OBJECTIVE Pseudoangiomatous stromal hyperplasia is a benign, localized form of stromal overgrowth with a probable hormonal etiology. Our purpose is to describe the mammographic, sonographic, and clinical features of this entity. METHODS A retrospective review of breast imaging studies and demographic information for seven patients identified as having pseudoangiomatous stromal hyperplasia was performed. The patients were chosen from a 1-year period during which 1661 breast biopsies were performed. RESULTS The patients were 36-61 years old. Six were premenopausal, and the one postmenopausal woman was on hormone replacement therapy. Three patients wee evaluated palpable breast lumps, and four were asymptomatic. All seven women had noncalcified masses that measured 1.1-11 cm and that were visible by mammography. The border characteristics were as follows: three masses were well circumscribed, two were partly circumscribed, and two were indistinct, probably because they were obscured by overlying parenchyma. Sonography was performed for five patients; lesions were visible in four. All four of these lesions were solid and circumscribed. Five of the masses had increased in size since earlier studies, and one palpable mass was found by physical examination to have grown over time. Two patients had a local recurrence of pseudoangiomatous stromal hyperplasia. Surgical excision was performed for three patients, large-core biopsy was performed for three patients, and both surgery and large-core biopsy were performed for one patient. CONCLUSIONS Pseudoangiomatous stromal hyperplasia should be included in the differential diagnosis of a circumscribed or partially circumscribed mass, especially in the premenopausal population. These masses often grow over time and can recur locally. Pathologic diagnosis of the lesion may be difficult unless the pathologist is aware of the presence of a mass lesion and appreciates the stromal changes characteristic of such a lesion.