Carcinomas of the bladder that resemble clear cell carcinoma of mullerian type are rare. Whether such neoplasms 1) arise from mullerian elements in the bladder and are histogenetically identical to the female genital tract cancer, 2) are a peculiar variant of vesical adenocarcinoma of nonmullerian derivation, or 3) represent a peculiar morphologic expression of transitional cell (urothelial) carcinoma with gland differentiation is often uncertain. We reviewed the clinical, conventional pathologic, and immunohistochemical features of 13 neoplasms with exclusive, or predominant, morphologic features of clear cell carcinoma. The 11 female and two male patients were 22-83 (mean 57) years of age. The clinical and gross features had no unique aspects. On microscopic examination the most common pattern, present in all cases, was tubulocystic, with a papillary pattern, present in six tumors and a predominant solid growth in one. Cells with abundant clear cytoplasm were conspicuous in nine tumors and hobnail cells were seen in eight. Four tumors showed focally recognizable patterns of transitional cell (urothelial) carcinoma in the available material. In five other tumors pseudostratified epithelium reminiscent of transitional epithelium was present focally. Endometriosis was present in two cases. In two other cases benign cysts focally lined by ciliated epithelium and surrounded by elastosis were interpreted as most likely mullerian. Immunohistochemistry was performed in 10 cases. All tumors stained for CA 125 (usually strong, ranging from focal to diffuse) and nine tumors stained for CK7 (usually strong and diffuse). CK20 was focally and weakly positive in four tumors and extensively positive in another. The same immunohistochemical panel was performed on 10 typical transitional cell carcinomas, 4 transitional cell carcinomas with gland differentiation, not otherwise specified, and 5 pure adenocarcinomas of the bladder (one of urachal origin). Minimal CA 125 positivity was seen in two transitional cell carcinomas. CA 125 staining was seen in the areas of gland differentiation in three of four transitional cell carcinomas and three of five pure adenocarcinomas but was focal in most cases. All transitional cell carcinomas and transitional cell carcinomas with gland differentiation showed extensive CK7 positivity. In contrast, only one of four positive pure adenocarcinomas showed >5% CK7-positive cells. Although all groups showed CK20 positivity, the percentage of CK20 positive cells was higher in pure adenocarcinomas. Prostate specific antigen was negative in all tumors. The cytokeratin immunoprofile of clear cell carcinomas of the bladder is closer to transitional cell carcinomas and transitional cell carcinomas with gland differentiation than pure adenocarcinomas arguing against an unusual form of adenocarcinoma. Our finding of CA 125 expression in bladder tumors of apparent urothelial origin contrasts with some studies that have regarded CA 125 expression as evidence for a mullerian origin. The frequency of gland differentiation in transitional cell carcinomas and the rarity of vesical endometriosis could be taken to suggest that these tumors are mostly of urothelial derivation, but the strong female preponderance in our series argues for a mullerian origin in at least some cases, and this is almost certain in the four cases with benign mullerian components. In the absence of endometriosis or conventional foci of transitional cell carcinoma, it may be impossible to determine whether a tumor with the morphology of clear cell carcinoma is of mullerian or transitional (urothelial) cell lineage, and at this time immunochemistry does not solve this problem.