Three women, 37, 44, and 46 years of age, had masses up to 4 cm in maximal size, which involved the posterior wall of the urinary bladder, treated by transurethral resection. Microscopic examination showed prominent involvement of the lamina propria and muscularis propria by tubules and cysts lined by müllerian-type epithelium; in two cases, the latter also focally replaced the urothelium and formed polypoid projections into the bladder lumen. The tubules and cysts were usually round to oval but occasionally exhibited prominent branching. In all three cases, the glands were predominantly lined by tubal-type epithelium (including ciliated cells, intercalated, and peg cells), that represented endosalpingiosis. Each case also contained a minor component of glands lined by columnar mucinous cells (endocervicosis). In one case, there was a minor component of typical endometriosis, and, in another, endometrioid epithelium was associated with prominent pseudoxanthoma cells in the adjacent stroma. On the basis of these three cases, striking endosalpingiosis of the urinary bladder, which has not been previously documented, is usually associated with endocervicosis, endometriosis, or both, and, in such cases, the designation "müllerianosis" is appropriate. Müllerianosis can be distinguished from adenocarcinoma and various non-neoplastic glandular lesions of the urinary bladder on the basis of a variety of architectural and cytologic features.