Twenty patients with xeroderma pigmentosum group A (XP-A) and 30 healthy controls were studied to determine the energy requirements for UV-erythema with monochromatic light between 255 and 340 nm. As test sites, unexposed skin on the upper back was selected for irradiation using a prism monochromator and a 1 kW xenon arc source. The minimal erythema doses (MEDs) were determined at each erythemal response peak, at 24 hours post irradiation in normal controls and at 72 hours in XP-A patients, respectively. Compared with normal subjects, XP-A patients had extremely low MEDs at all wavelengths tested. The erythema action spectrum showed 2 peaks at 265 and 290 nm in XP-A patients and the greatest effectiveness at 290 nm. These results indicate that XP-A patients should be protected strongly against not only UVB but also UVA, because the photo-hypersensitivity in XP-A patients has been extended from the UVB to UVA region.