SAN FRANCISCO - California has seven times as many residents as Colorado, but nearly nine times fewer medical marijuana users, at least on paper.

And as far as record-keepers know, the most populous state, home to the nation's first and most liberal medical marijuana laws, also has a smaller number of pot patients than Arizona, Hawaii, Michigan, Montana and Oregon.

If those statistics look off-kilter, they should. The reality is that no one knows how many people are legally using marijuana in California because the state - with hundreds of pot stores and clinics that issue medical marijuana recommendations - does not require residents to register as patients. Of the 16 states that allow the medicinal use of cannabis, it is one of only three without such a requirement.

Now, with California's medical marijuana industry laboring under a renewed federal crackdown that has forced many storefront dispensaries to close, a state lawmaker has recently introduced legislation that, if passed, would give authorities a much clearer count of the drug's bona fide consumer base.

Sponsored by Assemblywoman Nora Campos, a San Jose Democrat, the bill would require anyone who wants to claim a legal right to use marijuana for health reasons to apply for a county-issued identification card. Marijuana patients also would have to say if they plan to grow their own pot or to purchase it from a patient collective, and name the collective.

The changes are designed to make it easier for police and sheriff's deputies to identify who can legally consume and grow marijuana and who is using medical marijuana laws as a cover for illegal drug possession or dealing, said Randy Perry, the Peace Officers Research Association of California lobbyist who wrote the bill.

"We are not saying people shouldn't be smoking it or eating it. The people have spoken, and that's legal," Perry said. "We are simply trying to organize it a little bit so our law enforcement officers won't have to arrest people who can legally have it and won't have to confiscate their legally grown marijuana plants when there is a lot of crime and a lot of criminals they need to be going after."

California already has a state-run medical marijuana patient database and program under which counties are required to issue ID cards to eligible patients. The program was adopted by lawmakers in 2003 as a way to protect legitimate medical patients from arrest when caught with marijuana in their cars. The registry system was seen as a way to add a measure of control to California's voter-approved law seven years earlier decriminalizing marijuana for medical use.

The registry was made voluntary, however, and relatively few patients have signed up. The California Department of Public Health reports that during the fiscal year that ended last June, the state had only 9,637 valid card holders.

In Colorado, by contrast, the state with a medical marijuana regime most similar to California's but where patient registration and annual renewal is mandatory, the total number of patients holding valid ID cards as of December was 82,089. If California's patients were registering at that rate, there would be more than 615,000 of them.

Medical marijuana-related businesses are ubiquitous in parts of California. But just three cards were issued for every 100,000 residents last year, putting the state below every state with a mandatory patient registry. New Mexico, with one of the most stringent medical marijuana laws, issued 21 cards for every 100,000.

In California, records show the pot growing region known as Emerald Triangle - Mendocino, Humboldt and Trinity counties - had among the highest rates of registrations in the state.

Health department officials declined to discuss the registry's unpopularity, but the reasons for it are hardly a mystery. Although the system was set up with extensive privacy protections, such as identifying patients by numbers instead of names, many people are reluctant to enter personal information on a government database since marijuana still is illegal under federal law.

Medical marijuana advocacy groups have done little to dispel the fears, and some doctors who specialize in writing marijuana recommendations have fueled confusion by providing plastic ID cards that many users mistakenly assume offer the same protections as the county-issued ones until they are in a traffic stop, said San Diego criminal defense lawyer Melissa Bobrow.

"I can understand the reluctance of patients to go on this list, but at the end of the day you want to think of the reality and what the worst-case scenario is," said Bobrow, who represents pot patients facing drug charges.

The bill requiring California's pot patients to register is likely to meet fierce opposition from medical marijuana advocates, who have gone to court to block state and local laws limiting how many plants people can legally grow and regulations dictating where and how pot shops can operate.

Retired state Sen. John Vasconcellos, who sponsored the legislation creating the voluntary marijuana patient registry, predicted current lawmakers would be pre-empted from making the program mandatory, even if they approve the bill. The Legislature in his view cannot override voters who established at the ballot box that eligible patients only need a doctor's recommendation to be legal.

Vasconcellos - himself on the marijuana registry- said he opposes making it mandatory. But he expressed surprise that so few Californians have availed themselves voluntarily of the "free pass" the system was supposed to provide. He was incredulous that in his own Santa Clara County there were at least 150 pot shops a year ago but only 198 residents with current registrations.

"I'm proud to have my card," he said, "and I'm saddened people don't feel like they can trust the integrity of the governmental process."