Fenfluramine was administered to eight severely ill schizophrenic patients as an adjunct to neuroleptic drugs in a double-blind, multiple baseline design. A significant adverse effect of fenfluramine on psychopathology was detected through nurses' ratings, target symptom scales, the Brief Psychiatric Rating Scale, and time-sampled behavioral observations. Clinical deterioration was correlated with fenfluramine-induced reductions in blood serotonin levels and persisted beyond the point of discontinuation of fenfluramine.