The former chief of pediatrics at Fort Belvoir pleaded guilty yesterday to sexually assaulting a teen-age patient in an Alexandria health clinic in 1983.

Former Lt. Col. Arthur C. Andreasen, 40, sat silently in Alexandria Circuit Court as a prosecutor charged that the doctor sexually abused the boy since he was 13.

Assistant Commonwealth's Attorney Mark R. Voss said later that the physician had victimized other children. "We know for a fact that there were multiple victims," Voss said, noting that the doctor was under investigation by federal authorities.

Andreasen, a graduate of the University of Kentucky Medical Center, surrendered his medical license to Virginia authorities after he was charged with sodomy and forcible sexual assault in the June 5, 1983, incident. He faces up to 20 years in prison when he is sentenced June 2.

The former patient of Andreasen, now 15 years old, did not appear in court. Voss said the doctor used "force, threats, and intimidation" against the boy.

Neither Andreasen nor his lawyer, James M. Lowe, contested any of the evidence. The doctor declined to comment after the hearing.

Alexandria police said the teen-ager did not report the abuse until November 1985 because he was embarrassed and humiliated.

"Our primary concern now is to identify as many of the victims as possible because resources are available to help the other young males who may have been victimized by Dr. Andreasen," Voss said after the hearing. He said several victims were receiving counseling.

Federal prosecutors are investigating allegations of sexual misconduct against the doctor that occurred prior to the 1983 incident when he was working at Fort Belvoir. He worked there between 1980 and 1983.

The FBI, the Army's Criminal Investigation Division and the U.S. attorney's office in Alexandria also are investigating the Army's decision to allow Andreasen to resign from the service and enter private practice without informing civilian medical boards of an allegation of sexual misconduct against him.

The Army knew that at least one 14-year-old patient had alleged in a written statement that the doctor sodomized him and that Andreasen had given a qualified admission of the incident, according to federal investigators.

In 1984, the armed forces changed its rules, making it mandatory for all formal disciplinary actions against military doctors to be reported to the National Federation of State Medical Boards.

Shortly after the 15-year-old boy informed police about the incident at the Adolescent Counseling Service in Alexandria, Andreasen resigned from the Kaiser-Permanente health maintenance organization, where he worked after leaving the Army.