BACKGROUND AND PURPOSE Conservative treatment represents the current therapy for early-stage breast cancer. When risk factors for local relapse exist, a tumour bed boost is required. Retrospectively, we evaluated the prognostic factors influencing local recurrence (LR), overall survival (OS) and disease-free survival (DFS). MATERIAL AND METHODS After conservative treatment, 210 patients received a single-dose HDR brachytherapy (HDRBT) boost between June 1996 and December 2005. Mean age was 57 years; 75% had invasive ductal carcinoma. The most frequent surgery was lumpectomy (55.7%); 39.4% were G3, 18.6% intraductal component >25% and only 22% had negative margins. RESULTS With a mean follow-up of 85 months, at 5 and 10 years the OS was 93% and 88%, DFS 92% and 89%, and LR 3.6% and 5.3%, respectively. For LR, the risk factors were carcinoma in situ, N+ and involved margins, whereas for metastasis, the risk factors were T2 tumours, stage III, N+ and the presence of local recurrence. CONCLUSIONS HDR-BT boost in one fraction is an effective, simple and safe method for reducing LR. The outpatient setting and shorter treatment duration represent undeniable advantages.