Eight patients with mucinous ovarian tumors and pseudomyxoma peritonei were matched by their original ovarian tumor histology with eight tumor controls without pseudomyxoma peritonei. In each group, four tumors were benign, three were low malignant potential, and one was malignant. Cases compared with controls more frequently showed cyst rupture (3 vs. 2), periovarian adhesions (5 vs. 4), mucinous lesions of the appendix (4 vs. 0), goblet cells (8 vs. 4), and tumor necrosis (3 vs. 1). All cases displayed pseudomyxoma ovarii, which contained mucinous epithelial cells. Only one control exhibited pseudomyxoma ovarii, but it did not contain epithelial cells. In all cases, the peritoneal mucin contained epithelial cells. Two cases were noteworthy, each of which displayed a change in histology (benign to low malignant potential and benign to malignant) 6 years after the original ovarian tumor operation. One of these cases was not associated with pseudomyxoma peritonei until the first recurrence of the benign ovarian tumor. These data suggest that pseudomyxoma ovarii, with epithelial cells, may presage the development of pseudomyxoma peritonei and/or malignant recurrence.