DURING THE PERIOD MAY 1, 1974, TO MAY 1, 1975, THERE WERE 198 ASSAULTS AMONG 142 MEN CONFINED TO THE MAXIMUM SECURITY DIVISION OF THE PENETANGUISHENE MENTAL HEALTH CENTER, ONTARIO, CANADA. FREQUENCY OF ASSAULTS DID NOT VARY SIGNIFICANTLY OVER THE MONTHS OR OVER THE DAYS OF THE WEEK, BUT MOST ASSAULTS OCCURRED DURING THE WAKING HOURS. STAFF COULD PROVIDE NO MOTIVE IN 62.5 PERCENT OF THE CASES, BUT ONLY 8.3 PERCENT OF THE ASSAULTERS SAID THAT THEY HAD NO REASON FOR COMMITTING THE ASSAULT. TEASING BY ANOTHER PATIENT WAS REGARDED AS A CAUSE OF ASSAULT BY 11.5 PERCENT OF THE STAFF AND WAS STATED AS A REASON BY 26.8 PERCENT OF THE ASSAULTERS. A REFUSED REQUEST WAS REGARDED BY 1.6 PERCENT OF STAFF AS A CAUSE OF ASSAULT BUT WAS STATED AS A REASON BY 10.2 PERCENT OF THE ASSAULTERS. PROVOCATION BY STAFF MEMBER WAS CONSIDERED A CAUSE BY ONLY 1 PERCENT OF THE STAFF BUT WAS STATED AS A REASON BY 22.9 PERCENT OF THE ASSAULTERS. OTHER FACTORS VIEWED AS CAUSES OF ASSAULTS BY BOTH STAFF AND ASSAULTERS WERE OBJECTIONS TO RULES, HOMOSEXUAL APPROACHES, AND HALLUCINATIONS. IT WAS FOUND THAT 18 OF 142 PATIENTS ACCOUNTED FOR 61 PERCENT OF THE ASSAULTS. THE ASSAULTERS TENDED TO BE MEN WHO HAD BEEN MANAGEMENT PROBLEMS IN PSYCHIATRIC FACILITIES, AS OPPOSED TO THOSE REFERRED BY THE COURTS OR CORRECTIONS SYSTEM. THEY WERE SIGNIFICANTLY MORE LIKELY TO HAVE BEEN ADMITTED TO OTHER PSYCHIATRIC FACILITIES, WERE SIGNIFICANTLY LESS LIKELY TO HAVE HAD A CRIMINAL HISTORY, WERE LESS LIKELY TO BE REFERRED FOR INCOMPETENCY TO STAND TRIAL, WERE SIGNIFICANTLY YOUNGER AT TIME OF ADMISSION (26.39 YEARS, AS OPPOSED TO 31.59 YEARS FOR THE NONASSAULTERS), AND HAD POORER PREHOSPITAL ADJUSTMENT. VICTIMS WERE STAFF MEMBERS IN 66 PERCENT OF CASES. LOCKING UP THE ASSAULTER DID NOT DETER FURTHER ASSAULT BUT DID PROVIDE SAFETY FOR OTHERS ON THE WARD. THE PROBLEM OF TREATMENT FOR THE ASSAULTER IS DISCUSSED. REFERENCES ARE INCLUDED. (GLR)