Actinic keratosis (AK) and Bowen's disease (BD) are common patterns of in situ squamous cell carcinoma of the epidermis. In AK, atypical keratinocytes proliferate in the lower portion of the epidermis including the basal layer. In contrast, BD features atypical squamous cells in all portions of the epidermis but initially leaves basal cells in palisades along the basement membrane. To characterize immunohistochemically keratocyte proliferation in AK and Palisading Basal Cells (PBC) in BD, we stained microarray samples of 45 AK and 25 BD with Molecular Immunology Borstel (MIB-1). Subsequent immunostaining of full mounted sections examined 11 BD, 7 AK, and 4 examples of psoriasis for MIB-1 (as a proliferative marker) and p53 (as a cell cycle regulatory marker). AK stained for MIB-1 and p53 antibodies only in lower portion of epidermis and included the basal layer. BD with typical PBCs stained positive for both markers throughout the epidermis, except for the basal layer. Psoriatic biopsies stained positively for the 2 markers only in the basal and parabasal layers. Normal epidermis adjacent to the lesions in AK and BD biopsies stained sparsely in the basal layers. The correlation of different histologic patterns of epidermal involvement with different immunohistochemical patterns of stains argues for different cells of origin for BD versus AK. Lack of expression of proliferative antigens in palisading basal cells in BD provides evidence that PBCs are not the cell of origin for BD. Conversely in AK, expression of MIB-1 and p53 in basal cells argues that these cells play a role in histogenesis of AK.