Hamartoma or mastoma is a benign breast lesion which is unfamiliar to most clinicians. Five cases with this lesion are presented. It is a rather soft, sharply delineated lump which may cause a considerably asymmetry of the breasts. Radiologically it appears as a circumscribed tumour with homogenous or varying density and at operation the lump is easily enucleated. Histologically it contains normal or dysplastic mammary tissue. It is unilateral and benign. The lesion is now more frequently diagnosed because of increased use of mammography and can be mistaken for a neoplasm. It is therefore important that clinicians and radiologists are acquainted with it.