Immunohistochemical staining of proliferating cell nuclear antigen (PCNA) was performed in skin from patients with various malignant and nonmalignant skin diseases using anti-PCNA monoclonal antibodies. The malignant diseases included squamous cell carcinoma (SCC), adult T lymphotrophic leukemia (ATL), mycosis fungoides, malignant melanoma and malignant lymphoma, and the nonmalignant diseases included severe treatment-resistant atopic dermatitis (AD), psoriasis vulgaris, verruca vulgaris, and others. The percentage of PCNA-positive cells (the labeling index, LI) was highest for the malignant diseases (56.5+/-7.1%). The LIs for severe treatment-resistant AD, psoriasis, and verruca vulgaris were also significantly higher than those for the normal control or nonlesional skin of the patients. The PCNA LIs were, however, not significantly elevated in eczema and contact dermatitis. The high PCNA LIs in severe AD and psoriasis vulgaris were considerably lower in the skin improved by treatment. Labeling with Ki67, a nuclear protein expressed in cycling cells, was also performed in skin from subsets of each patient group. The results were very similar to those found with PCNA labeling. PCNA-positive cells were found throughout the dermis as well as the basal layer in the malignant diseases, whereas they were found only in the basal layer in the nonmalignant diseases. The results suggest that in human skin diseases, the extent of staining for PCNA, which is a cofactor of DNA polymerase-delta and is essential for cell proliferation, correlates with the extent to which the disease is treatment-resistant. In addition, our findings suggest that the PCNA LI and distribution of PCNA-positive cells in the skin may be helpful in the early diagnosis of skin malignancies.