Introduction Angiosarcoma consists only 0.04% of all breast malignancies and has a poor prognosis. This is the first reported case of an angiosarcoma arising in the non-operated breast after primary irradiation for occult breast cancer. The patient underwent mastectomy, surviving disease free for 6 years. Presentation of case A 73-year-old woman with a past history of irradiation of the non-operated left breast complained of skin thickening and crust formation on the left nipple 8 years post-irradiation. Considering the clinical history and radiological studies, recurrent cancer was suspected and biopsy was performed. However, no proof of malignancy was obtained. As clinical symptoms continued to advance, informed consent was obtained and mastectomy was performed. Histological examination of the surgical specimen revealed angiosarcoma. Discussion In this case, angiosarcoma occurred after radiation on a non-operated breast. Preoperative diagnosis was not achieved even with two cytology specimen and one biopsy. Each showed only fibrosis and inflammatory changes. The background breast tissue inflammation should have been caused by radiation. Marked fibrosis and the rather small number of sarcoma cells in the breast tumor in this case may be why bioptic diagnosis was difficult. Kaplan-Meier analysis of 60 Japanese breast angiosarcoma patients showed significantly better prognosis in patients with a tumor 2 cm or smaller. Conclusion Angiosarcoma may occur in the non-operated breast, post irradiation. The potential difficulties of diagnosing angiosarcoma against background fibrosis should be kept in mind. Initial radical surgery currently represents the only effective treatment for improving survival in these patients.