To identify a sensitive marker for extramammary Paget's disease and to identify histochemical and immunohistochemical features that suggest occult pelvic cancer in patients with extramammary Paget's disease, we retrieved all cases between 1983 and 1992 with a Standardized Nomenclature of Medicine code of extramammary Paget's disease in the Vanderbilt University Medical Center (Nashville, Tenn) surgical pathology archives. All were stained for alcian blue/dPAS (periodic acid-Schiff), mucicarmine, AE1/AE3, cytokeratin (CAM 5.2), cytokeratin (CK) 7, CK 20, carcinoembryonic antigen (CEA), orthokeratin, prostate-specific antigen, and S-100. Sixteen cases (2 men, 14 women) were retrieved. Two had pelvic malignancies: one rectal adenocarcinoma and one transitional carcinoma. Only CK7 marked all cases. Mucins were sensitive but focal, a potential problem in small biopsy specimens. The transitional tumor had a unique staining profile (CEA- and mucin-negative). CK20 strongly marked Paget cells associated with rectal cancer; its presence suggests a large bowel lesion but is not specific. No case expressed prostate-specific antigen; its presence in a man suggests prostatic carcinoma.