Personalities of 60 users of illicit LSD and 60 matched nonusers were compared by the results of the Minnesota Multiphasic Personality Inventory (MMPI). Eighty-eight per cent of the users' MMPI protocols were judged abnormal as compared to 54 per cent of the nonusers‘. In addition, the users' mean scores were significantly greater (p< .05) than nonusers for the neurotic triad, the psychosis classification, and the behavior disorder category. It was suggested that the relationships between elevated MMPI scales and LSD usage should be viewed as correlational, since it was impossible to determine whether the measured personality deviations of the users predated LSD use.