Acute leukemia occurred in a user of several hallucinogenic drugs (lysergic acid diethylamide [LSD], mescaline, marihuana, and amphetamines). The leukemic leukocytes of the patient contained a Ph1-like chromosome; lymphocytes transformed with phytohemagglutinin did not show chromosome breaks. A Ph1-like chromosome has only rarely been reported in either acute leukemia or in cultured lymphocytes of individuals ingesting LSD.