More than 35 Maryland doctors in the past two decades have been convicted of sex crimes, drug charges, theft, Medicaid fraud and other criminal violations without losing their rights to practice medicine.

Thirteen doctors in all, nearly half of them psychiatrists, have been disciplined for criminal or unethical sexual activity during the past two decades, according to commission records. One was suspended for five years. All the others were reprimanded or placed on probation and allowed to continue practicing under certain conditions.

Francesco B. DiLeo, a psychiatrist and former assistant professor of clinical psychiatry at the University of Maryland, was placed on supervised probation last summer after the commission found he had indulged in hallucinogenic drugs with his patients and had sex with one of them during a session at his home "institute."

DiLeo, who is now working under close supervision at Springfield State Hospital, a state mental facility, said in an interview that he had been trying to break new ground in the use of psychedelic substances for psychiatric treatment.

A family practitioner in Montgomery County was convicted and served time in prison for a 1971 assault on a woman patient. County police were monitoring the doctor on closed circuit television when he went to the patient's apartment, gave her an injection that knocked her out, then undressed and assaulted her, according to testimony at the trial and commission documents. The doctor was acquitted of more serious rape charges.

The commission found that the doctor's actions that evening "did not follow any usual, customary, acceptable therapeutic regime," and it placed him on supervised probation for three years.

The doctor, contacted recently at his home in Silver Spring, declined to discuss the case and said he has retired from medical practice.

The commission routinely allows doctors convicted of Medicaid fraud to continue practicing medicine, frustrating criminal prosecutors in the attorney general's office. Over the 20-year life of the commission, two doctors have lost their licenses on grounds that included Medicaid fraud; 19 others have been reprimanded or placed on probation.

Similarly, the commission often allows doctors who have been involved in illegal drug distribution to continue practicing medicine. The Federation of State Medical Boards, a national clearinghouse of information on disciplinary actions against physicians, recommends automatic revocation of a license when a disciplinary board finds a doctor guilty of selling drugs illegally.

Dr. Samuel B. Rosser, for example, a Silver Spring surgeon and an assistant pediatrics professor at Howard University Hospital, was convicted in 1979 in federal court in the District of illegally writing prescriptions for 7,000 tablets of Dilaudid, a heroin substitute.

The commission placed Rosser on probation for three years and ordered him to provide at least 416 hours of free medical care to the community. The District of Columbia Commission on Healing Arts, which, like the commission in Maryland, has staffing problems and a case backlog, dropped its charges against Rosser in part because of their delays in pursuing the case.

Howard allowed Rosser to maintain his privileges, and he continues to practice there. He said in an interview that he wrote the prescriptions at the request of a friend who said she needed them for patients in a nursing home she ran. Rosser said he believes he was made a scapegoat by law enforcement authorities. In 1980, the commission found that Dr. Walcott Gibson, a family practitioner in Marlow Heights, sold prescriptions for the tranquilizer Valium to an undercover state trooper, even after the trooper told him she intended to resell the Valium on the street. Gibson was placed on three years' probation, during which he was ordered to submit to a quarterly review of his practice. He was barred from prescribing narcotics for one year.

Gibson said in an interview that he had prescribed drugs such as Valium to patients trying to break addictions to harder drugs. His lawyer, Joseph A. DePaul, said Gibson was a victim of police entrapment.