Primary retroperitoneal mucinous tumors (PRMTs) are uncommon neoplasms occurring almost exclusively in women. PRMTs are divided into mucinous cystadenoma (MC), mucinous borderline tumors or tumors of low malignant potential (MLMP), and mucinous carcinomas (MCas). In this retrospective study, we present the clinicopathologic features of 18 such cases, the largest series to date. All patients were women, ranging in age from 20 to 63 years (mean 38.6 y). All except 2 patients presented with an enlarged mass during a routine examination or by self-palpation. All tumors were located exclusively in the retroperitoneum, with histologic or clinical confirmation of the lack of ovarian involvement. The tumors ranged from 7 to 26 cm (mean 13.2 cm). The gross appearance was variable: unilocular cyst with a thin wall (4 cases), predominantly cystic with papillary areas or nodule(s) (8 cases), multiloculated cyst with or without nodules (1 case each), and predominantly solid with cystic areas (4 cases). Histologically, there were 2 cases of MC, 7 of MLMP (7 cases; 3 of them with intraepithelial carcinoma and 1 with microinvasion), and 9 of MCas (9 cases, 5 of them associated with MLMP and 1 associated with MC). Three of the MCas had areas of anaplastic or sarcomatoid carcinoma whereas 1 had an associated sarcoma. Immunohistochemical studies were performed in 6 cases. Cytokeratin 7 was diffusely positive in all cases studied, whereas cytokeratin 20 and cytokeratin 17 were focally positive in 4 and 2 cases, respectively. All patients underwent surgical resection of the entire tumor. Two patients with MCa and sarcoma or sarcomatoid carcinoma received chemotherapy. Follow-up was available in 16 cases, ranging from 1 to 148 months (mean 40 mo, median 22 mo). Two patients died of disease at 5 and 9 months; both had MCa with anaplastic carcinoma or sarcoma. Three patients with MCa were alive with disease at 14, 26, and 58 months. The remaining patients were alive with no evidence of disease. In this study, MLMP and MCa were more common than MC. PRMTs seem to be nonaggressive neoplasms, except in cases containing anaplastic carcinoma or sarcoma.