The mental state of 20 psychotic men with high urinary cannabinoid levels on admission to a psychiatric hospital was compared with that of 20 matched cannabis-free controls. All patients underwent toxicological analysis to exclude the presence of alcohol and other exogenous agents. Cannabis levels were measured by a semiquantitative enzyme immunological technique and mental state was assessed by the use of the Present State Examination (PSE), once shortly after admission and again 7 days later. The cannabis group showed significantly more hypomania and agitation and significantly less affective flattening, auditory hallucinations, incoherence of speech, and hysteria than did the controls. Clouding of consciousness was absent in most cannabis patients. After 1 week the cannabis group showed marked improvement (particularly in the psychotic syndromes), whereas the controls remained virtually unchanged. There was no significant difference in amount of medication received between the two groups. Our data suggest that a high intake of cannabis may be related to a rapidly resolving psychosis manifesting with marked hypomanic features, though often presenting as a schizophrenia-like illness. © 1982.