In this study 55 paraffin embedded samples defined as Bowen's disease or bowenoid papulosis were investigated with antibodies against S 100 protein and keratins (KL 1). S 100-positive cells were quantified and related to defined section area of the epidermal compartment by computer-assisted image analysis. The density of S 100-positive cells was compared with normal skin and was particularly related to growth patterns and keratinization of the different lesions under study. S 100-positive dendritic cells were found to be reduced overall in bowenoid lesions when compared with normal skin. Lesions with high counts of S 100-positive dendritic cells most frequently showed a solitary growth pattern with highly conserved architecture and differentiation and no tendency to stromal invasion. In contrast, cases with low counts of S 100-positive cells very often showed multifocal development, a high degree of architectural disturbance and dedifferentiation. In this group, stromal invasion (cases of invasive carcinoma associated with Bowen's disease) was seen more often. Interestingly, this latter group of cases also revealed a peculiar keratin pattern. Frequently, the basal cell layer was decorated with KL 1 antibody, which usually recognizes only suprabasaly located keratinocytes. No differences between Bowen's disease and bowenoid papulosis were found in terms of densities of S 100-positive dendritic cells and keratin pattern. In our experience, extragenital Bowen's disease and genital Bowen's disease can not be distinguished on purely morphological grounds or with the immunocytochemical approach presented here. Interestingly, when employing in situ hybridization with HPV 16 probes three of seven samples of genital Bowen's disease harboured HPV 16 DNA, whereas six cases of extragenital disease were negative.