The determination of the enzymatic activity of URO-D in erythrocytes is the screening method used for differentiation between hereditary and non-hereditary forms of porphyria cutanea tarda (PCT). The aim of the present work was to establish the relative frequencies of the symptomatic and hereditary forms by the determination of the URO-D enzyme in the PCT patients who were regularly treated at the Centre for Porphyrins in our Institute. In the course of this work we also examined the statistical properties of the distributions of both normal and porphyric subjects, so as to be able to suggest values for discriminating between normal subjects and the various types of porphyric subjects.