We measured uroporphyrinogen decarboxylase (UROD) activity in erythrocyte lysates obtained from 40 consecutive patients with porphyria cutanea tarda (PCT) without selection for family history. Enzyme determinations indicated that 28% of the patients had abnormally decreased UROD activity in erythrocytes; this finding did not always correlate with family history. Two siblings with PCT and normal erythrocytic, but abnormally decreased hepatic UROD activities, were encountered. This finding suggests that familial PCT may occur not only with decreased erythrocyte UROD activity, but also with a normal UROD activity in erythrocytes.