Twenty-two cases in which mucinous tumors of the appendix were associated with mucinous tumors of the ovary are reported. The patients ranged from 23 to 83 (average 49) years of age and usually presented with increasing abdominal girth. The appendiceal and ovarian tumors were synchronous in 21 cases. Laparotomy typically disclosed large cystic ovarian tumors that averaged 16 cm in diameter and were usually multiocular, an appendix that was usually dilated and covered with mucus, and abundant intra-abdominal mucus. The ovarian tumors were bilateral in seven cases. The ovarian and appendiceal tumors were typically similar histologically, with features similar to those of ovarian mucinous cystadenomas and cystadenomas of borderline malignancy. In most of the ovarian tumors, mucin dissected through the ovarian stroma (so-called pseudomyxoma ovarii). Eight of the 20 patients with follow-up information were well when last seen, but the duration of follow-up was 3 years or less in six of them. Two patients died of pseudomyxoma peritonei 4 and 5.5 years after presentation. One patient died of a myocardial infarct shortly after laparotomy for recurrent pseudomyxoma peritonei at 11 years. The remaining patients had definite or probable recurrent or residual disease but were alive at the time of the last follow-up information. The typical synchronous presentation of the ovarian and appendiceal tumors, their histologic similarity, the frequency of bilaterality of the ovarian tumors, the predominance of right-sided ovarian involvement, and the usual presence of mucin and atypical mucinous cells on the ovarian surfaces all point toward the probable secondary nature of the ovarian tumors.