Carcinoma of the breast in children is an extremely rare disease, only 44 patients have been reported in the world literature. We report on a case of breast cancer in a 9-year-old girl, who was admitted to the hospital because of a four-month history of a painless mass in the right breast. Clinical examination and mammography resulted in a strong suspicion of malignancy. The histology corresponded to juvenile secretory carcinoma, the histological type without the characteristics of prepubertal cancer cases. Electron microscopy demonstrated intracellular large vacuoles and acini filled with secretory material. The mitotic index was low, the highest measured value was 4 mitotic figures per 10 high power fields corresponding to 2.78 mitotic figures per mm2 of epithelium in the microscope field. Flow cytometry showed that the neoplastic cells were diploid and had a low S-phase fraction. A simple mastectomy including axillary evacuation was performed. No lymph node or distant metastases were found. Postoperative radiation treatment or chemotherapy were not administered. Our patient has been in follow-up for six years without recurrence. In this case simple mastectomy with axillary evacuation seems to have given a cancer-free development into adulthood. Earlier reports have also shown a favourable outcome in these tumours.