In a series of 103 psychotic patients with evidence of drug abuse, the following facts came to light. 1. A definitive diagnosis was made in 94%. In 6% of these cases the diagnosis remained unclear even after having been admitted for a second time. 2. Seventy-four percent (72 of 97) of the patients, who abused various drugs, usually in combination had organic psychoses, and 26% (25 of 97) were diagnosed as schizophrenics. 3. Of 36 patients, who suffered psychotic episodes due to the abuse of cannabis, or LSD, or heroin, 21 (= 58%) were schizophrenics. 4. In those cases showing organic psychoses, thought insertion, thought withdrawal, and thought broadcasting were not found. However all other first- (and second-) rank symptoms (Kurt Schneider) of schizophrenia were found. Perhaps this might be of help as a differential criterion. 5. The fact of drug abuse at the outset of a schizophrenia is dependent on the schizophrenic symptomatology. The use of and attitude to the drug as well as the experiencing of the drug's effects changes in the further course of the psychosis. This last point might also be of value in the differential diagnosis.