Porphyria cutanea tarda (PCX) is characterized by cutaneous symptoms in association with hepatic accumulation and urinary excretion of mainly uro- and heptacarboxyporphyrins. 24 PCX patients excreting urinary porphyrins between 1.9 and 5.8 μmol/24 h (normal < 0.2) were treated with chloroquine at a dose of 125 mg twice a week. During the first phase of therapy urinary porphyrin excretion transiently increased 1.1- to 2.9-fold indicating a phase of mobilization. A slight initial elevation was also found regarding the activities of serum amino-transferases reflecting the hepatotoxic effect of chloroquine. Xhe clinical symptoms disappeared after a period ranging from 2 to 6 months, and after an average of 12 months the porphyrin excretion in all patients had returned to nearly normal values.