The men show up on foot, on bicycles, by van or car.

Before entering the sky-blue door at the rear of the vast office plaza, they carefully look around to make sure no one is watching them go in.

Behind the door, authorities say, is a Medicare racket. Manassas Medical Center in Doral has billed the taxpayer-funded insurance program for therapy that patients with HIV or AIDS haven't received.

Among the men stepping in on a June morning is Alexander McCray, who regularly stops at Manassas and dozens of Miami-Dade clinics.

McCray, a 40-year-old Opa-locka resident, admits he pockets thousands of dollars in kickbacks in exchange for giving the clinics his Medicare number to bill the agency for phony HIV infusion treatments. Manassas is among hundreds of Medicare-licensed clinics in South Florida that defraud the system with fake HIV-drug claims, according to federal claim records and authorities.

The scams are especially outrageous because HIV infusion therapy, which entails intravenous drips of medication to boost a patient's immune system, has been replaced almost everywhere but South Florida by more effective antiretroviral drugs taken orally. Yet Medicare has continued to allow the outdated HIV infusion services and to pay hundreds of millions of dollars for the treatments because the agency still considers them ``reasonable and necessary.''

That policy leaves some veteran HIV/AIDS specialists dismayed.

''There isn't any reason today to be giving any infusion therapy,'' said Dr. Margaret Fischl, who specializes in immunology at the University of Miami's School of Medicine.

Dozens of South Florida clinics -- along with others popping up in Orlando, Tampa and St. Petersburg -- are still submitting billions in Medicare claims for HIV infusion therapy, but the total bills have dropped slightly because of stepped-up criminal prosecutions of operators, physicians and billing companies during the past year, according to federal court and claims records.

Medicare, however, continues to pay these clinics hundreds of millions of dollars in reimbursements for HIV infusion therapy, according to Health and Human Services' inspector general.

And that, in turn, subsidizes kickbacks to patients, authorities said.

''Far and away, the largest number of individuals committing Medicare fraud are the patients,'' Kirk Ogrosky, deputy chief of the fraud section at the Department of Justice, said at an American Bar Association conference in Fort Lauderdale in May. ``The department must prosecute patients in order to stop the demand for kickbacks.

``If you don't stop the demand in the community by patients seeking cash, the durable medical equipment and infusion clinics are a never-ending cycle of fraud with a new one popping up every time we shut one down.''

Indeed, HIV infusion scams at local healthcare clinics are happening in plain sight, all over South Florida. And they wouldn't be able to exist without ``professional patients.''

CRACK HABIT

Enter McCray, a small-time criminal with a 15-year history of drug-possession arrests, according to court records. He was last arrested in April on charges of trying to buy cocaine on Miami's streets to feed his crack habit, records show.

McCray received his Medicare disability number as an HIV-positive patient on April 26, 2001.

He has sold his Medicare ID number to private healthcare clinics in Miami-Dade for $150 to $300 a visit -- as often as three times a day, three times a week, over seven years, according to federal claim records.