Three cases belonging to xeroderma pigmentosum (XP) complementation group E were analyzed clinically and photobiologically. The three Japanese patients were a 50-yr-old female (XP80TO), a 42-yr-old female (XP81TO), and a 41-yr-old female (XP82TO). They were assigned to complementation group E by the cell hybridization study. All showed lowered minimal erythema doses between those of normal Japanese and XP group A subjects at wavelengths of 280, 290, and 300 nm of monochromatic ultraviolet (UV) light. Patients XP80TO and XP81TO, but not patient XP82TO, showed a delayed peak reaction at 48 h to UV erythema. All fibroblast strains from these patients had a reduced level of 40%-44% unscheduled DNA synthesis (UDS) after irradiation with 10 J/m2 of 254 nm UV. Primary cultured epidermal cells from these patients exhibited a relatively low level of UDS (ie, 38%-51% of normal epidermal cells). All of the group E fibroblast strains were twice as sensitive to 254 nm UV killing [n (extrapolation number) = 1.3-1.8, Do (mean lethal dose) = 2.2-2.8 J/m2)] as normal fibroblasts (n = 1.5, Do = 5.0 J/m2). All of the above group E patients had mild XP symptoms, but not neurological abnormalities, at the fifth decade of age. Patients XP80TO and XP81TO had developed skin malignancies (patients XP80TO developed three basaliomas; patient XP81TO developed two basaliomas) at the ages of 46 and 41 yr, respectively.