A causative role of UV light in the development of melanocytic neoplasms has often been suggested. In order to investigate the short-term effects of UV light on melanocytic nevi. the morphological and immunohistochemical changes in nevi after a single UV irradiation are studied in 12 nevi from 10 patients and compared with the nonirradiated part of the same nevus. After irradiation more melanocytes above the dermal-epidermal junction are observed in seven nevi. simulating a melanoma in situ in three nevi. Moreover, a marked increase in the expression of HMB-45 is found after irradiation in all investigated nevi, indicating an activation of the melanocytes and active melanosome formation. The metabolic activity correlates with the ultrastructural findings, which show a large cytoplasm, hypertrophic Golgi apparatus, abundant mitochondria, and an increased number of mclanosomes of different stages. One week after irradiation, no increase in the proliferative activity of the melanocytes is found. The morphological and immunohistochemical changes after one low dose of UV irradiation should be considered in the differential diagnosis of pigmented skin lesions. The UV-irradiated nevus should be added to the list of so-called simulators of malignant melanoma.