The carcinogenic effect of UVA radiation (from Philips black light tubes filtered through a 2 mm-thick glass plate to eliminate the radiation below 320 nm) was studied in 7 groups of 25 lightly pigmented hairless mice. Irradiation with a moderate daily dose of combined UVB and UVA for 3 months induced a tumor incidence of 0.22 after 58 weeks. When the combined UVB and UVA irradiation was followed by filtered UVA for 2, 4, or 6 months, the tumor incidence was marginally significantly increased to 0.42, 0.48, and 0.50 (p less than 0.05), respectively. However, irradiation with the moderate dose of combined UVB and UVA induced a slight but not significantly lower tumor incidence as compared to UVB alone (0.22 vs 0.30, p greater than 0.1). UVA alone induced no tumors. It thus appears that in hairless mice initially exposed to a combination of UVB and UVA, subsequent continued irradiation with UVA increases tumor incidence. While only marginally statistically significant, tumor incidence in these animals seems to increase with duration and hence total UVA exposure. Furthermore, it is suggested that the photoaugmentative carcinogenic effect of UVA irradiation from unfiltered UVA bulbs can be reduced by attenuating the shorter wavelengths of the radiation.