Nineteen patients (10 men, 9 women) with pseudomyxoma peritonei were studied to determine the site of origin of the disease and the nature of associated ovarian tumors. A primary appendiceal mucinous neoplasm was found in 16 of the 17 patients with an evaluable appendix. One woman's appendix had intramural and serosal mucinous deposits without neoplastic epithelial cells. The appendiceal tumors were classified as mucin-producing adenoma ("mucinous cystadenoma") in 13 patients (seven women, six men), intramucosal adenocarcinoma associated with mucin-producing adenoma in one man, and invasive adenocarcinoma associated with mucin-producing adenoma in one man and one woman. Five of the nine women had cystic mucinous ovarian neoplasms; each also had an appendiceal mucinous neoplasm (adenoma in four, invasive adenocarcinoma with adenoma in one). The ovarian neoplasms had histologic features resembling a borderline (low malignant potential) mucinous tumor in four and a mucinous cystadenoma in one; all five ovarian tumors also had features of pseudomyxoma ovarii. Mucinous implants were also on the ovarian surface of the contralateral ovary in four of the five women with ovarian tumors and in the other four women without ovarian tumors. The intraperitoneal mucus deposits contained neoplastic mucinous epithelial cells in 16 patients (eight men, eight women) and were acellular in three (two women, one man). Of 17 patients with more than 6 months of follow-up, 12 (seven women, five men) were alive after postoperative intervals of 7 to 147 months, including three with known residual disease. Five (three men, two women) died of disease 16 to 60 months after initial operation, including two patients with appendiceal carcinoma. Acellular intraperitoneal mucus appeared to be a favorable prognostic feature. We conclude that (a) the appendix is the primary site of origin of pseudomyxoma in the vast majority of cases in both men and women, and (b) the associated mucinous ovarian tumors are most likely secondary neoplasms resulting from incorporation of implanted mucus and neoplastic mucinous epithelial cells of the pseudomyxoma peritonei.