Abstract: Pseudoangiomatous stromal hyperplasia (PASH) is a benign mesenchymal proliferative lesion of the breast. In 2005, only 109 cases had been reported since its initial description in 1986 by Vuitch et al. Our 24 cases represent one of the largest series to be reported from a single institution. We retrospectively reviewed data from 2004 to 2010 of patients diagnosed with PASH by surgical excision or image-guided biopsy. All pathological specimens were reviewed by a single pathologist. The samples were stained for estrogen and progesterone receptors (ER and PR), CD34, and the lymphatic marker D2-40. All but one of 24 (96%) patients presented with breast masses either on imaging or clinically. Fourteen of the 24 patients (58%) were diagnosed on surgical excision, 10 (42%) diagnosed with core needle biopsy, and five (20%) were diagnosed using both techniques. The tumors ranged in size from 0.3 cm to 7.0 cm. All women except two were premenopausal or perimenopausal at diagnosis. Nineteen samples were available for hormonal receptor staining and of these 18 of 19 (95%) were ER or PR positive. PASH was diagnosed in two men, a transgender male on hormones and the other with gynecomastia. The patients’ ages ranged from 18 to 86 years old. In addition to PASH other benign histopathological findings include stromal fibrosis and atypical ductal or lobular hyperplasia. Imaging revealed no distinguishing feature for PASH with benign histology. One patient had synchronous ductal carcinoma in-situ (DCIS). Patients were treated with local excision or observation. This study suggests that PASH is primarily a diagnosis of premenopausal and perimenopausal women. Our series supports a hormonal basis for its development due to the positive staining for hormonal receptors. Management is conservative surgery for larger masses with careful observation being an option in patients not at high risk for breast cancer.