• Fifteen patients who developed prolonged psychotic reactions following psychotomimetic drug use (probably primarily LSD) were followed up 1.9 to 5.8 years later. Two patients had committed suicide. Approximately half of the patients had a relatively good outcome and half did poorly. Aspects of the initial clinical picture that correlated with outcome measures are discussed. The possibility is considered that vulnerability to a prolonged psychotic reaction following psychotomimetic drug use may be related to a genetic vulnerability to illnesses in the manic-depressive/schizo-affective spectrum. In some instances this vulnerability may implicate central serotonergic neuronal systems.