The usual phenomena of cannabis intoxication include experiences which in a non-intoxicated state would be considered as psychiatric symptoms. These can be distinguished from adverse reactions to cannabis, the commonest of which is an acute anxiety state. Acute psychotic episodes can also follow ingestion of the drug but are infrequent. These can be classified as acute confusional states and episodes occurring in clear consciousness. Neither presentation consistently shows enough specific features to warrant the diagnosis of 'cannabis psychosis' as a distinct clinical entity. The evidence that cannabis has a causative role in chronic psychotic or affective disorders is not convincing, although the drug may modify the course of an already established illness. Further controlled studies would be required to clarify the aetiological significance of the drug in these conditions.