Six cases of a distinctive ovarian lesion associated with sclerosing peritonitis were found in patients 13 to 76 years of age. Five patients presented with abdominal swelling; on examination, three were found to have unilateral or bilateral pelvic masses, one had ascites and evidence of small bowel obstruction, and one had ascites alone. The sixth patient presented with small bowel obstruction. At operation, 1 to 9 L of ascitic fluid were found in five patients. Five patients with bilateral ovarian enlargement underwent bilateral oophorectomy or, in one case, bilateral ovarian wedge resection. The sixth patient had unilateral ovarian enlargement treated by unilateral oophorectomy. All had fibrotic thickening of the peritoneum, most prominently involving the omentum and small bowel, four at initial operation and two at a second operation performed shortly thereafter. One or more episodes of small bowel obstruction subsequently occurred in three of the five patients with follow-up. One woman died of pulmonary embolism 2 months after her initial operation; four were alive without clinical evidence of disease 8 months to 6 years postoperatively. Gross examination revealed unilateral (one case) or bilateral (two cases), predominantly solid ovarian tumors 12 to 31 cm in maximum dimension in three cases and slight enlargement and nodularity of both ovaries in the other three cases. Microscopic examination of the ovaries revealed a cellular proliferation of spindle cells with focal differentiation into nests of luteinized or partially luteinized cells; one case also had occasional foci of sex cord elements. The process was confined to the ovarian cortex in the three cases with slight ovarian enlargement. Residual ovarian follicles were entrapped by the proliferation in three cases, but were prominent in only one of them. Additional features included brisk mitotic activity, predominantly in the spindle cells, in three cases, and striking edema in the three cases with significant ovarian enlargement. The ovarian lesions were interpreted as luteinized thecomas, although a closely related hyperplasia with luteinization, instead of early neoplasia, cannot be excluded in the three cases with only slight ovarian enlargement (and in one, prominent entrapment of follicles). The peritoneal process consisted of a variably cellular proliferation of fibroblasts and myofibroblasts separated by collagen, fibrin, and occasional chronic inflammatory cells. These cases and two similar cases from the literature suggest a unique association between some luteinized thecomas (or a closely related proliferative lesion of the ovary) and sclerosing peritonitis, although the relation between the ovarian and peritoneal processes is presently enigmatic.