Skin exposure to UVB radiation deprives the antigen-presenting function and OKT6 monoclonal antibody-binding characteristics of Langerhans cells. Decrease of Langerhans cell population could be relevant to immune surveillance disturbance in the UV-exposed skin. Patients with xeroderma pigmentosum exhibit a thousandfold higher risk for sunlight-induced skin cancers than patients with normal skin, and also have various defects in cellular immunity. Therefore, studies of numerical and structural changes in epidermal Langerhans cells of patients with xeroderma pigmentosum after UVB irradiation compared with normal subjects may contribute to understanding the role of antigen-presenting cells in photocarcinogenesis. The effect of UVB radiation on OKT6+ Langerhans cells was studied in epidermal sheets obtained from irradiated normal subjects (36 and 49 years old) and subjects with xeroderma pigmentosum complementation group A (21 years old), complementation group D (32 years old), and variant (35 and 60 years old). Langerhans cell densities in chronically sunlight-exposed skin were remarkably reduced in patients with xeroderma pigmentosum group A but only slightly in those with xeroderma pigmentosum variant and in normal subjects compared with covered skin. Structural changes were substantial in Langerhans cells of chronically exposed patients with xeroderma pigmentosum group A but fewer in subjects with other xeroderma pigmentosum groups and in normal subjects. A single irradiation of three times the minimal erythema dose induced a large reduction of Langerhans cells from 3 to 7 days in all subjects. However, subsequent reappearance and return to preirradiated levels was delayed more in patients with xeroderma pigmentosum group A than in those with xeroderma pigmentosum variant and normal subjects. These results indicate (1) an essential role for excisional repair in the UVB-induced depletion, recovery, and maintenance of the epidermal Langerhans cell population, and (2) a possible, but not confirmed, relationship between depletion of Langerhans cells and earlier photocarcinogenesis in xeroderma pigmentosum.