A marked discrepancy between mild and late clinical features and a nearly complete absence of erythrocyte uroporphyrinogen decarboxylase activity (Ery-UROD activity) was observed in a case of inherited porphyria cutanea tarda. The entity and time of appearance of clinical features, the onset of clinical symptoms after exposure to contributing factors, the effectiveness of phlebotomies and heterozygosity of the mother alone for uroporphyrinogen decarboxylase (UROD) deficiency were typical for familial porphyria cutanea tarda (F-PCT), whereas the extremely low UROD activity was peculiar to hepatoerythropoietic porphyria (HEP). These observations indicate that: 1) Ery-UROD activity may not always be useful to discriminate between F-PCT and HEP; 2) Ery-UROD activity does not always correlate with clinical symptoms; 3) in inherited UROD deficiency, the genetic defect may be heterogeneous. Finally, the observed discrepancy may provide additional evidence for the existence of tissue-specific isozymes.