The natural history of ultraviolet radiation-induced herpes simplex labialis and response to therapy with peroral and topical formulations of acyclovir. The lips of 196 patients with a history of sun-induced herpes labialis were exposed to experimental ultraviolet radiation (UVR) and treated with acyclovir (ACV) or placebo at different times and by different routes. Of 98 placebo recipients, 39 (40%) developed 43 lesions inside or within 10 mm of the irradiated zone. The temporal distribution of lesions was bimodal. 11 (26%) occurring within 48 h (immediate) and 32 (72%) 2-7 days after UVR exposure (delayed). Prophylactic peroral ACV begun 7 days before or 5 min after UVR prevented the development of the delayed but not the immediate lesions (P less than .001). When peroral ACV was started 48 h after UVR, delayed lesions developed but were less severe (P = .01-.05). Prophylactic topical ACV begun 5 min after UVR did not reduce lesion frequency or severity. ACV therapy can be efficacious, but some rapidly developing lesions are unresponsive to treatment. This suggests that more than one process may contribute to the pathogenesis of herpes labialis. 