The reactions to monochromatic radiation of the skin of ten patients with xeroderma pigmentosum were investigated, and eight were abnormal. The abnormalities consisted of papular and vesicular reactions and delay in the development of the minimal erythema dose reaction with wavelengths principally in the 290-320 nm range. Three patients were too young for full action spectra to be obtained but of those patients in whom all wavelengths were tested only one showed a reaction to radiation above 320 nm and this was at 340 nm only. In only one patient was the minimal erythema dose at 300 nm at 24 h lower than normal. In six patients the repair synthesis of deoxyribonucleic acid after ultraviolet radiation was estimated. Reduced levels were seen in five but repair was normal in one patient. The patient with normal repair also had normal reactions to monochromatic radiation. The abnormal reaction of the skin to artificial radiation and the abnormal deoxyribonucleic acid repair synthesis may enable the diagnosis of xeroderma pigmentosum to be made at a very early age. In one of the patients the diagnosis was made at the age of 6 months in a child with photosensitivity but with no other clinical signs of the disease. It is suggested that, by making the diagnosis as early as possible and by protecting the skin from natural sunlight, cutaneous malignancies may be prevented.