Porphyria cutanea tarda is thought to result from an inherited deficiency of uroporphyrinogen decarboxylase (EC 4.1.1.37) in some patients. Present methods for determining uroporphyrinogen decarboxylase activity are time consuming, so we examined the pattern of porphyrins formed from porphobilinogen by hemolysates as a possible marker for hereditary porphyria cutanea tarda. After the hemolysates are incubated with porphobilinogen, the porphyrins are converted to their methyl esters and examined by liquid chromatography, with fluorometric detection. The porphyrinic patients examined, and some of their relatives, showed a characteristic pattern of porphyrin production, with high uroporphyrin/coproporphyrin and (uroporphyrin + heptacarboxylic porphyrins)/coproporphyrin ratios, at least partly ascribable to increased uroporphyrinogen I synthetase (EC 4.2.1.8) activity in patients' hemolysates, and also to a relative deficiency of uroporphyrinogen decarboxylase. Examination of the pattern of porphyrins produced from porphobilinogen by hemolysates is a suitable technique for detecting asymptomatic individuals with porphyria cutanea tarda.