The nature of the distinctive ovarian lesion often associated with sclerosing peritonitis, initially considered a variant of luteinized thecoma in the paper describing this phenomenon, remains uncertain, as does its long-term prognosis. We describe the features of 27 cases, including immunohistochemical analysis of 13 cases. Sclerosing peritonitis was documented in 25 cases. Patients ranged in age from 10 months to 85 years, and typically presented with abdominal distension and pain with ascites and sometimes bowel obstruction. The ovarian lesions, clinically bilateral in 24 cases, ranged from 2 to 31 cm and often had a striking cerebriform aspect. Microscopically, mitotically active spindle cells with weakly luteinized cells, variable edema, and entrapped follicles were typical. The spindle cells were focally positive with calretinin in 2 cases, CD56 in 2, AE1/3 in 4, smooth muscle actin in 12, and desmin in 8 cases, and negative with alpha-inhibin, epithelial membrane antigen, beta-catenin, CD34, and transforming growth factor-beta, with focal nuclear positivity for estrogen receptor in 5 and progesterone receptor in 11 cases. Luteinized cells were positive with alpha-inhibin, calretinin, and/or CD56. The peritoneal lesions were strongly positive with AE1/3 and exhibited focal weak or moderate positivity with estrogen receptor or progesterone receptor in 4 of 8 cases each. Follow-up in 20 cases (mean: 5.9 y) disclosed no evidence of spread of the ovarian lesion, but 3 patients died of sclerosing peritonitis. The findings fail to allow definitive classification of the ovarian lesions, and we prefer at present to retain their current designation as a subtype of luteinized thecoma, but to allow for the possibility of a non-neoplastic nature, feel it reasonable to have the designation "thecomatosis" as a parenthetical alternative. We have documented for the first time that sclerosing peritonitis is not invariably associated with the distinctive ovarian pathology present in these cases.