In humans, epicutaneous application of a universally sensitizing dose (2000 micrograms) of dinitrochlorobenzene (DNCB) to skin exposed to 4 consecutive daily doses (144 mJ/cm2) of ultraviolet-B radiation (UVB) induces contact hypersensitivity (CH) in approximately 56% of normal, adult individuals (UVB-resistant--UVB-R), but not in the remaining 44% (UVB-susceptible--UVB-S). In patients with biopsy proven basal/squamous cell cancer, the frequency of the UVB-S trait exceeds 90%, indicating that this phenotype may be a risk factor for sunlight-induced skin cancer. Since many patients with recurrent herpes labialis complain that lip lesions are precipitated by acute sun exposure, we wondered whether the UVB-S trait might be associated with this recurrent disease. A group of 31 volunteers was selected, each with a history of numerous episodes of labialis secondary to reactivated herpes simplex virus-1 infection. Subjects were questioned carefully concerning factors, including sun exposure, thought to be important in precipitating lip lesions. Each individual was then subjected to the UVB plus DNCB protocol. When forearm skin of these individuals was assayed for CH after 30 days, 20 (65%) proved to be UVB-S (approximately 1.5 times the expected frequency), while the remainder displayed vigorous DNCB-specific CH. A strong history of sun-induced recurrent herpes simplex labialis did not predict the UVB phenotype. A subset of these subjects was exposed to 2 MEDs of UVB to their faces. None of the UVB-R subjects developed recurrent herpes labialis while 6 of 8 UVB-S subjects developed recurrent lesions.(ABSTRACT TRUNCATED AT 250 WORDS)