A Montgomery County Circuit Court judge yesterday dismissed an assault case because of a recent Maryland Court of Appeals ruling requiring that criminal cases be tried within 120 days of the defendant's first court appearance.

It was the first time a case in the Washington suburbs had been dismissed because of the speedy trial ruling.

The dismissal in the case, however, was made "without prejudice" - a legal distinction which means prosecutors may bring new assault charges against defendant John Cunningham for the same alleged offense.

Defense attorneys unsuccessfully tried to win dismissal in three other Montgomery County cases and one Prince George's case this week under the appeals court ruling.

Although the effect of the appeals court ruling has been mininal thus far, prosecutors in Prince George's say that as many as 65 criminal cases could be affected - 25 percent of the total number of criminal cases pending.

In Baltimore City, however, up to 200 cases could be jeopardized, involving such crimes as robbery, murder, rape and arson.

Montgomery court officials have not yet compiled any statistics on the number of cases that might be affected.

The case dismissed yesterday involved an inmate at the Montgomery County Detention Center who was charged with three counts of assault for allegedly beating a fellow inmate with a mop wringer last November.

The court proceedings against Cunningham, 31, date back to December 1978. His trial was originally scheduled for March 15 but was postponed twice.

Under the appeals court ruling, cases may be tried beyond the 120-day limit if they have been delayed because of an "extraordinary cause." The court, however, failed to define what constitutes an extraordinary cause.

But yesterday, Montgomery Circuit Judge Philip M. Fairbanks ruled there was no extraordinary cause for the postponements in the Cunningham case.

A spokesman for the state's attorney's office said the case was delayed once because prosecutors were unable to obtain a psychiatric evaluation of Cunningham before the first trial was scheduled to start. Cunningham had pleaded not guilty by reason of insanity.

The trial was postponed a second time when an expert witness for the defense was unable to appear, according to Cunningham's attorney, Frank Coviello.

The state's attorney's office may appeal Fairbanks' ruling or may seek a new indictment without bothering to appeal, a spokesman for that office said.

Attorneys say one of the problems with the appeals court ruling is its vagueness.

Earlier this week, for example, another Montgomery Circuit Court judge ruled that a delay in the completion of a defendant's psychiatric evaluation did constitute extraordinary cause, and allowed the trial to proceed despite the violation of the 120-day deadline.