Skin phototesting and cellular sensitivity studies were performed in a patient with xeroderma pigmentosum (XP) complementation group E (XP80TO) at the ages of 50 and 55 years. She showed a reduced minimal erythema dose at both ages, but the dose at age 55 was much lower than that at age 50 when tested with monochromatic ultraviolet (UV) light (280, 290 and 300 nm). The cellular sensitivity to UVC (254 nm), UVB and UVA and UVC-induced unscheduled DNA synthesis were examined using fibroblasts obtained by skin biopsy at the ages of 50 and 55 (XP80TO-1 and XP80TO-2, respectively). DNA synthesis was similar in both cell lines. XP80TO-2 cells were more sensitive to UVB cytotoxicity than XP80TO-1 cells in both the dividing and quiescent phases, but both cell lines exhibited a similar sensitivity to UVC and UVA. These results suggest that the in vitro cellular sensitivity to UVB may correlate with the clinically observed erythema reaction. Further, the results suggest that some XP complementation group E cases at least may show an increase in photosensitivity in vivo and in vitro with aging.