Between 1986 and 1996, 35 patients with a diagnosis of hamartoma of the breast were seen at the Mayo Clinic. One patient had two lesions. The mean age was 50 years (range 21-86 years). Hamartomas were clinically identified as a palpable lump in 18 cases (11 were detected by the patient and 7 by a physician). The other 18 were identified mammographically. Twenty-four lesions were in the left breast and 12 were in the right breast; 39% were located in the upper outer quadrant. Mammographically most hamartomas were ovoid, and the lesions were well circumscribed. Sonographically they were all solid, but 24% showed cystic areas. Pathologically the mean greatest diameter was 3.2 cm (range 1.0-7.5 cm). All but one lesion showed circumscription. The mean percentage of fibrous tissue was 78% (range 5-95%), fat 13% (range 0-95%), and epithelium 9% (range 1.0-60%). Calcifications were seen in four lesions. Ductal hyperplasia was present in 27% and adenosis in 70% of lesions. Twelve percent of patients had coexistent fibroadenomas.