A new method of thin-layer chromatography was used for the study of urinary porphyrins in 42 porphyric patients (27 cases of porphyria cutanea tarda (PCT), 5 cases of porphyria variegata, 4 cases of acute intermittent porphyria, 2 cases of hereditary coproporphyria and 4 cases of erythropoietic protoporphyria), 21 of their clinically normal relatives and 5 controls. The results are compared to previously published data and discussed for the diagnosis of the porphyrias. If urinary porphyrin pattern seems sufficiently pathognomonic for PCT, it appears often unable to allow exact diagnosis of the acute porphyrias; faecal studies and sometimes enzymatic determinations are necessary.