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In order to confirm the genetic character of porphyria cutanea tarda (PCT), the quantitative and qualitative porphyrin excretion from 56 unrelated PCT patients and 259 relatives was analyzed by a sensitive fluorimetric thin-layer chromatographic technique. Porphyrin excretion abnormalities were observed in 111 (35.24%) of the 315 subjects studied. Of the 259 relatives, 55 (21.24%) suffered from manifest (24 cases) or subclinical (31 cases) PCT. The relatives from the older generation or a generation similar to the propositi were more frequently affected than those from a younger generation. A clear family incidence was observed in 32 families, while PCT was apparently limited to the propositi in the remaining 24. It is discussed whether these latter families correspond to the so-called 'sporadic' type of PCT or include porphyric gene carriers lacking biochemical expression of the disease. While the measurements of the activity of the defective enzyme (uroporphyrinogen decarboxylase) for the genetic research of PCT turned out to be impracticable in hepatic tissue and contradictory in erythrocytes, our study confirms that the familial character of this disease may be revealed by the chromatographic analysis of the porphyrin excretion pattern. [ABSTRACT FROM AUTHOR]