The term ductal adenoma has been recently introduced to describe a solid benign lesion of breast ducts. This study describes the clinical, morphologic, and immunohistochemical features of 15 cases of ductal adenoma. Ductal adenomas are usually single, occasionally multiple, lesions occupying medium- and large-sized breast ducts. They may occur in women of all ages, although the majority of patients are 60 years of age or greater. Ductal adenomas usually present clinically as breast lumps which may mimic carcinoma; less commonly, they are associated with nipple discharge. Patients in this series showed no family or previous history of breast disease and had uneventful follow-up after local excision. Despite often showing worrying pseudoinfiltration and cytologic atypia, the immunohistochemical demonstration of a myoepithelial layer and intact basement membrane around the tubules was clear evidence of the benign nature of the lesions. We conclude that most ductal adenomas evolve by sclerosis of benign intraduct papillary lesions, although processes similar to sclerosing adenosis and, possibly, duct ectasia may contribute to the pathogenesis of a proportion of cases. It is hoped that a wider appreciation of the entity of ductal adenoma will reduce the diagnostic uncertainty that continues to surround these and related lesions.