OBJECTIVE With effective screening programmes, the global incidence of in situ ductal carcinoma of the breast has risen to 15%, representing 20 to 30% of the mammographically detected T0 tumours. Prognosis is generally good in these tumours, but treatment has in the past, paradoxically, relied on extensive surgery. We report our experience with conservative radiosurgery performed in such cases from 1980 to 1990. METHODS There were 86 patients with a mean age of 52 years. Initial TNM classification was T0 = 57, T1 = 17, T2 = 9 and Tx = 3. All underwent limited surgery (quadrantectomy: 17; lumpectomy: 69) and 49 had axillary dissection. All received breast irradiation with scar boost (46-50 Gy + 10-14 Gy with classical fractionation). Thirty one postmenopausal women received adjuvant tamoxifen. The quality of the resection was good in 77 cases, doubtful or incomplete in 9. In one case axillary nodal involvement was found. The histological subtype was clearly identified in 63 cases. With a median follow-up of 58 months, 3 local relapses occurred (3.4%), at 27, 48 and 52 months respectively. One was in situ and two invasive. All were clinical lesions (2 T1 and 1 T2); two had incomplete or doubtful excision and all received less than 60 Gy on the tumour bed. All had mastectomy. Two are alive and well but one developed multiple metastasis. Five other women had subsequent surgery for suspicion of local relapse but all had benign disease. One developed contralateral disease 20 months later. Two women died subsequently due to a second cancer. CONCLUSIONS These results confirm the importance of the excision quality and suggests a possible dose-effect in the control of in situ ductal carcinoma by radiotherapy. The recent results of the B-17 NSBAP trial also conclude that the radiosurgical conservative treatment, for limited in situ ductal carcinoma, is a reasonable alternative to mastectomy.