Ovarian-type surface epithelial tumors are rare in the testis and paratestis, with most reported examples being serous. There is little information on mucinous tumors, nine of which are described in this report, applying criteria and terminology used in the ovary. The patients were 44-69 years of age (median 64 years) and presented with masses in the testis (four) or paratestis (five). Eight tumors were cystic (median size, 3.5 cm) and contained gelatinous material; one (a paratesticular carcinoma) appeared as thickening of the tunica vaginalis. Two were classified as mucinous cystadenomas (both paratesticular), six as purely or predominantly borderline tumors (four, testis; two, paratestis; one had intraepithelial carcinoma and one microinvasive carcinoma) and one (paratestis) as mucinous carcinoma. The cystadenomas were composed of endocervical-like cells, but intestinal-like cells typified the borderline tumors and carcinomas. Cyst rupture with mucin dissection into the stroma, inflammation, and dystrophic calcification with ossification were common. No tumor was associated with intratubular germ cell neoplasia, unclassified type or with teratomatous elements. One patient with carcinoma died shortly after presentation with peritoneal spread; autopsy disclosed no other potential primary site. The follow-up (1.8-12 years) in all other cases was uneventful. Mucinous tumors of the testis and paratestis resemble their ovarian counterparts, exhibiting the same morphologic spectrum, from benign to borderline to malignant, and having both endocervical-like and intestinal features. These tumors may derive from mesothelium by the process of mullerian neometaplasia, from mullerian remnants or from the mucinous epithelium of a teratoma. The older age of the patients, lack of nonmucinous elements, and absence of intratubular germ cell neoplasia, unclassified type suggest that, if of teratomatous origin, the teratoma is different from the usual type. Clinical features are important to exclude metastasis, particularly in cases of carcinoma and, to a lesser extent, in tumors of borderline type.