The existence of hereditary porphyria cutanea tarda must be supported by chemical and clinical investigations capable of discriminating this porphyria from porphyria variegata, because the clinical symptoms may overlap. On the basis of such investigations (normal urinary excretion of deltaALA and of porphobilinogen; urinary excretion of large amounts of 8- and 7-carboxyl porphyrins; faecal coproporphyrin and X porphyrin fractions may be increased) we have been able to classify 14 cases, out of 200 cases of PCT that we have observed in the last 7 years, as hereditary PCT. The 14 patients belong to 5 different families: two members in each of the first three families, 3 members in the fourth, and 5 members in the fifth. In this last family heredity is bilateral.