THE STUDY SUBJECTS CAME BEFORE THE COURTS OF ESSEX COUNTY, N.J., AS THE RESULT OF A 1975 NEW JERSEY SUPREME COURT DECISION REQUIRING THAT NGI'S RECEIVE ESSENTIALLY THE SAME TREATMENT AS CIVILLY COMMITTED PERSONS. UNDER THE LAW, NGI'S MUST RECEIVE AN INITIAL COURT HEARING AND SUBSEQUENT PERIODIC HEARINGS REGARDING THEIR PRESENT MENTAL ILLNESS AND DANGEROUSNESS. BASED ON DATA ON THE NEW JERSEY NGI'S, THE STUDY EXPLORES FOUR AREAS: (1) THE TYPE OF DEFENDANT LIKELY TO BE ACQUITTED ON INSANITY GROUNDS; (2) THE CIRCUMSTANCES IN WHICH RELIANCE ON THE INSANITY DEFENSE IS WISE; (3) FACTORS THAT INFLUENCE RELEASE AFTER AN NGI FINDING; AND (4) THE APPROPRIATENESS OF INSANITY ACQUITTALS. CONTRARY TO THE COMMON BELIEF THAT ONLY DEFENDANTS ACCUSED OF SERIOUS VIOLENT CRIMES DEFEND ON INSANITY GROUNDS, THE STUDY POPULATION INCLUDED A FEW PERSONS CHARGED WITH LESSER OFFENSES (LEWDNESS, LARCENY, POSSESSION OF A CONCEALED WEAPON, SALE OF ILLICIT DRUGS). TWENTY-SEVEN PATIENTS HAD NEVER BEFORE BEEN HOSPITALIZED FOR MENTAL ILLNESS, WHILE 19 HAD. THIRTY-NINE PATIENTS HAD NEVER BEEN CONVICTED OF A CRIME. FACTORS INFLUENCING THE ADVISABILITY OF AN INSANITY DEFENSE INCLUDE THE CRIME COMMITTED, THE AVAILABILITY OF OTHER DEFENSES, THE PATIENT'S PSYCHIATRIC HISTORY, AND THE WILLINGNESS OF THE DEFENDANT TO RELY ON THE INSANITY DEFENSE, WITH ITS HAZARDS OF LENGTHY CONFINEMENT AND COURT-IMPOSED CONDITIONS AFTER RELEASE. UNLESS THE CRIME CHARGED IS SERIOUS OR UNLESS THE DEFENDANT IS A MULTIPLE OFFENDER LIKELY TO DRAW A LENGTHY SENTENCE IF CONVICTED, DEFENSE ATTORNEYS SHOULD CONSIDER THE INSANITY DEFENSE ONLY AS A LAST RESORT. BECAUSE THE MAJORITY OF PATIENTS STUDIED OBTAINED DISCHARGE AS A RESULT OF THEIR COURT HEARINGS, IT WAS DIFFICULT TO ISOLATE STATISTICALLY THOSE FACTORS INFLUENCING THE COURT'S DECISION TO RELEASE. HOWEVER, TWO FACTORS SEEM TO INFLUENCE THAT DECISION: (1) IF THE CRIME WAS HOMICIDE, THE PATIENT IS LIKELY TO BE RETAINED IN THE HOSPITAL LONGER, REGARDLESS OF HIS MENTAL CONDITION; AND (2) IF THE PATIENT'S LAWYER IS NOT EXPERIENCED IN MENTAL HEALTH MATTERS, RELEASE IS LESS LIKELY. OTHER FACTORS--THE HEARING PROSECUTOR'S ATTITUDE, WHETHER THE SAME JUDGE PRESIDES AT BOTH TRIAL AND HEARING, THE PATIENT'S PAST HISTORY, ETC.--SEEM TO HAVE LESS EFFECT. STUDY FINDINGS POINT TO THE CONCLUSION THAT A SIZABLE NUMBER OF INSANITY ACQUITTALS IN ESSEX COUNTY ARE ERRONEOUS BECAUSE THE DEFENDANTS CLEARLY WERE NOT MENTALLY ILL AT THE TIME OF THE CRIME. PROSECUTORS, DEFENSE COUNSEL, AND JUDGES ARE URGED TO INSIST ON MORE THOROUGH SCRUTINY OF INSANITY DEFENSES. NO SUPPORTING DATA ARE INCLUDED. (LKM)