A third of male inbred CFW/R1 mice in a breeding colony developed subcutaneous, bilateral, perineal masses determined to be cystic bulbourethral glands. The masses developed in mice between 4 and 15 months of age. After development of these perineal masses, diseased males were unable to produce offspring. Gross examination revealed the masses impinging on the scrotum and displacing the testes into the inguinal canal. The perineal masses were paired, membranous, translucent cysts, 6 to 10 mm3, attached to the bulbocavernosus muscle and connected to the pelvic urethra by way of a non-patent duct. The cysts contained a clear to tan, minimally cellular, viscous fluid with high mucus content, as documented by examination of Wright Giemsa-stained cytologic preparations. Histologic examination of hematoxylin and eosin-stained sections revealed cystic tubuloalveolar glands surrounded by striated muscle and lined by a single layer of pyramidal cuboidal to columnar epithelial cells with pale, basophilic, lacy cytoplasm and round, basal, condensed nuclei. These gross and histopathologic findings were consistent with cystic dilatation of the bulbourethral gland.