The diameter of collagen fibrils was studied in a patient with porphyria cutanea tarda in a specimen obtained from the sclerodermatous skin of the back. A bimodal distribution of the diameter of the fibrils was observed with maxima at 33.5 nm and 68.7 nm. The mean diameter was 67 nm with a standard deviation of 10.6 nm. These results are similar to those obtained previously in localized scleroderma (morphoea).