Seventy thecomas of the ovary were evaluated to ascertain their histopathologic spectrum. The tumors occurred over a wide age range (average 49.6 y). Presentation in the form of pelvic or abdominal pain was uncommon, but postmenopausal bleeding was relatively frequent. All the tumors were unilateral, ranging up to 22.5 cm (average 4.9 cm) in greatest dimension. They were typically intact, uniformly solid, and yellow. Microscopic examination usually showed a predominant diffuse growth but was altered to varying degrees by hyaline plaques (37 cases), nodular growth (20 cases), calcification (20 cases), and keloid-like sclerosis (12 cases). Forty percent of the tumors had a minor component of fibroma. Reticulin stains typically showed an investment of single cells. The tumor cells characteristically had ill-defined cytoplasmic membranes and distinctive pale gray cytoplasm. Two tumors had degenerative so-called bizarre atypia. Fifteen tumors had nuclear grooves, but they were rarely conspicuous. The differential diagnosis is primarily with other sex cord-stromal neoplasms, particularly sclerosing stromal tumor, microcystic stromal tumor, steroid cell tumor, and adult granulosa cell tumor. The nodules of the first have a more heterogenous morphology than the uniform cell type of thecomas, and microcystic stromal tumors are distinguished because of microcysts and differing character of the tumor cells. Steroid cell tumors also have contrasting cytoplasmic features. Granulosa cell tumor with a prominent thecomatous component is the most clinically important differential and is largely solved by thorough sampling. Our experience indicates a relatively distinctive appearance of thecomas, which contrasts with the lipid-rich character often emphasized in the literature. Awareness of this and a spectrum of other findings should enable accurate interpretation of an almost invariably benign tumor.