Bowen's disease is a premalignant dermatosis comprised of a clonal proliferation of atypical keratinocytes in the full thickness of the epidermis. To elucidate the relationship between the alteration of the p53 tumor suppressor protein and cell proliferation rate, we immunohistochemically examined the expression of proliferating cell nuclear antigen (PCNA) and p53 protein in 30 cases of Bowen's disease. All the cases exhibited the full-thickness distribution of PCNA-positive cycling cells in the lesional epidermis. Quantitation of PCNA staining by image cytometry revealed a mean labeling index (LI) of 75.1 +/- 20.3. p53 expression was detected in 13 cases (43%). Expression was diffuse (p53 LI > 50) in 9 cases, but focal (p53 LI < 30) in the other four. The mean PCNA LI of p53 diffusely positive cases was significantly greater than that of both p53 focally positive and p53 negative cases (89.3 +/- 10.1 vs 62.7 +/- 21.2, and 70.5 +/- 21.6; p < 0.01, respectively). These findings suggest that a high-level accumulation of p53 protein results in a more increased cell proliferation in Bowen's disease.