To the Editor.— In two of our patients who were suffering from photosensitivity, thiourea could be identified as the sensitizer. Report of Cases.— Case 1.—A 51-year-old white man developed photosensitivity three weeks after having started work with a photocopying machine. Fiery redness and swelling developed on the skin that was exposed to light, accompanied by burning sensations; even indoor light caused complaints. In light tests, the skin showed an increased sensitivity mainly to ultraviolet-A light (UV-A). Photopatch tests showed that the photocopy paper that was used by the patient in his work made his skin strongly reactive to light. The manufacturer of the paper kindly supplied us with data on the chemical composition of the paper and with samples of the 12 compounds involved. In photopatch tests with the separate compounds, thiourea turned out to be the cause of the severe reactions of the skin to light. Thiourea is used