Transitional cell carcinoma (TCC) of the ovary is a recently recognized subtype of ovarian surface epithelial-stromal cancer, and studies of its morphology are few. As a result, the criteria for its diagnosis and spectrum of its morphology are not clearly established. One hundred consecutive consultation cases of ovarian carcinoma with a pure or partial transitional cell pattern (excluding malignant Brenner tumor) diagnosed between 1989 and 2001 were evaluated for the frequency of various pathologic features and the relation of TCC to other surface epithelial-stromal carcinomas. The women were 33 to 94 years of age (mean, 56 years). A total of 47 tumors were stage I, 21 stage II, 31 stage III, and 1 stage IV; 13% of the stage I tumors and 41% of tumors of all stages were bilateral. The tumors ranged from 3.0 to 30 cm in greatest dimension (mean, 10 cm); 60% of them were solid and cystic, 24% solid, and 16% cystic. TCC was the exclusive or predominant component in 93% of the tumors and showed undulating (93%), diffuse (57%), insular (55%), and trabecular (43%) patterns. In four tumors with an insular growth, the pattern focally mimicked a Brenner tumor. Necrosis was present in 57% of the cases. Features that were seen in the tumors that in aggregate produced a relatively consistent appearance were "punched out" microspaces (87%), often the size of Call-Exner bodies, large cystic spaces (73%), and large blunt papillae (63%). Features that were sometimes seen, usually as a focal finding, included slit-like fenestrations (49%), bizarre giant cells (35%), small filiform papillae (18%), gland-like tubules (17%), squamous differentiation (13%), and psammoma bodies (4%). In 23 cases, TCC was a component of a mixed epithelial carcinoma, the additional components being serous adenocarcinoma in 16, endometrioid in 5, mucinous in 1, and clear cell carcinoma in 1. The tumor cells of the TCC component often were relatively monomorphic; 6% of the tumors were grade 1, 43% grade 2, and 51% grade 3. The nuclei were oblong or round and often had large single nucleoli (69%) or longitudinal grooves (48%). The cytoplasm was typically pale and granular but was rarely strikingly clear or oxyphilic. TCC of the ovary usually occurs in pure form but is also common as a component of a surface epithelial carcinoma of mixed cell type. In either situation, TCC has a constellation of architectural and cytologic features that readily distinguish it in most cases from other types of ovarian cancer. Recognition of these features will lead to a more consistent diagnosis of this tumor and aid in determining whether it has distinctive clinical features, particularly with regard to its behavior.