Fourteen cases of bowenoid papulosis were studied both clinically and histologically and were compared with 22 cases of Bowen's disease to find histologic differences between the two diseases. Bowenoid papulosis was prevalent in young, sexually active adults, consisted of multiple erythematous or hyperpigmented papules, and located mostly on the penis and vulva. The main histologic characteristics of bowenoid papulosis were an orderly background of keratinocyte maturation of the epidermis with monotonous pattern, the frequent presence of 'skip area', and the lesser degree of cytologic atypia than seen in lesions of Bowen's disease. In addition, mitotic figures more than 5 per x400 filed, formation of keratin pearl, and spindling of keratinocytes, which have not been described in other articles yet, are considered very important findings for the diagnosis of bowenoid papulosis. It is our opinion that bowenoid papulosis can be reliably distinguished from Bowen's disease by light microscopic features. Paraffin-embedded samples of bowenoid papulosis, Bowen's disease, and normal skin were investigated with antibodies against S 100-protein to relate the number of intraepidermal Langerhans cells and the disease. Langerhans cells were found to be reduced when compared with normal skin. No significant differences between Bowen's disease and bowenoid papulosis were found in density of Langerhans cells.