From a population of 111 patients with chronic methamphetamine (MAP) psychosis, who were treated at ten mental hospitals during the past 3 years, 21 patients were selected for study. Sixteen patients who experienced MAP psychosis again used MAP one or more times after long-term abstinence and experienced acute exacerbation of a paranoid psychotic state which was almost identical to the initial psychotic episode. Four of these patients relapsed following a single MAP reuse of an amount less than that initially used, and one relapsed without evidence of MAP reuse. In eight patients, small doses of neuroleptics, e.g., 3 mg per day of haloperidol, prevented the acute provocation of a psychotic state by MPA reuse. Subsequently, three of these relapsed into a psychotic state following MAP reuse without concurrent haloperidol medication. The clinical data were compared with animal experiments which indicate that chronic MAP use can induce a long-term susceptibility to sensitization to MAP. The positive prophylactic effect of small doses of haloperidol on the acute exacerbation may suggest the participation of dopaminergic supersensitivity as a mechanism for the paranoid psychotic state.