We report the clinical, morphologic, immunophenotypic, and ploidy findings of seven cases of serous borderline tumor of the paratestis. Mean patient age was 56 years (range, 14-77 years), and the clinical presentation was that of a testicular mass. Tumors ranged in size from 1 to 6 cm (mean, 3.5 cm). Six tumors arose from the tunica albuginea, and two of these tumors were intratesticular. One tumor arose from the tunica vaginalis. Serous borderline tumor of the paratestis is histologically identical to its ovarian counterpart. The tumors were cystic with numerous intracystic blunt papillae lined by stratified epithelial cells with minimal to mild cytologic atypia. Psammoma bodies were present in two cases. In all cases, the neoplastic cells stained strongly and diffusely for cytokeratin 7, estrogen receptor, and CD15, and six of seven cases were positive for progesterone receptor and MOC-31. The cells did not stain for cytokeratin 20, carcinoembryonic antigen, calretinin, and HER2/neu. Proliferative activity, as assessed by MIB-1 staining, ranged from 1.3% to 10% (mean, 5.5%). Five of six tumors were diploid, and one was tetraploid. Patients were treated by radical orchiectomy and followed up from 4 months to 18 years (mean, 48 months; median, 8.5 months). No recurrences or metastases occurred. Serous borderline tumor of the paratestis is morphologically and immunophenotypically identical to ovarian serous borderline tumor. To date, no serous borderline tumor of the paratestis reported in the literature or in our series has recurred or metastasized after resection.