To test the diagnostic specificity of reduced red-cell uroporphyrinogen decarboxylase activity for porphyria cutanea tarda, we measured enzymic activity in 29 normal subjects and 65 patients with various forms of porphyria. Only patients with porphyria cutanea tarda had subnormal enzymic activity. Patients with acute intermittent porphyria, erythropoietic protoporphyria, variegate porphyria and hereditary coproporphyria had normal or slightly elevated activities. The enzymic activity in normal persons and patients with porphyria cutanea tarda did not differ according to sex. Reduction of iron stores did not alter the enzymic activity in porphyria cutanea tarda. We conclude that reduced red-cell uroporphyrinogen decarboxylase activity is a specific and intrinsic defect in porphyria cutanea tarda; measurement of this enzyme is a reliable diagnostic test for this disease.