This study assesses the incidence, histogenesis, and significance of eccrine sweat duct involvement in Bowen's disease (BD). In a review of 96 cases of BD, four showed eccrine duct involvement on hematoxylin and eosin-stained histologic sections. One case was analyzed for deoxyribonucleic acid (DNA) ploidy by using computerized image analysis on Feulgen-stained slides. Sections were also stained immunohistochemically, using antibodies to carcinoembryonic antigen (CEA), gross cystic disease fluid protein (GCDFP), and S-100 protein, and for cytokeratins (CAM 5.2, AE 1/3). Our results showed that, in BD, (a) the eccrine sweat ducts can be extensively involved by atypical cells, (b) the atypical eccrine duct cells had an aneuploid DNA pattern, and (c) the atypical eccrine duct cells were negative for CEA, GCDFP, and S-100 protein but were positive for cytokeratins. We conclude that (a) the frequency of eccrine duct involvement in BD is relatively low (approximately 4 to 9%), (b) the aneuploid DNA pattern makes a benign squamous metaplasia unlikely, (c) the immunohistochemical results exclude coincidental Paget's disease or carcinoma of eccrine sweat glands, (d) the involvement of eccrine sweat ducts may represent a direct extension of the neoplastic epidermal keratinocytes, and (e) this process may have practical implications in the recurrence of superficially treated cases of BD.