BACKGROUND The risk of ipsilateral breast tumour recurrence (IBTR) following breast conservation surgery (BCS) for invasive breast cancer (IBC) and radiotherapy is dependent on patient-, tumour- and treatment-related variables. In the Cambridge Breast Unit, breast conserving surgery has been performed with a target radial margin of 5 mm for IBC, in combination with 40-Gy hypofractionated (15 fractions) breast radiotherapy, since 1999. METHODS An audit was performed of cases treated between 1999 and 2004. A total of 563 patients underwent BCS for invasive breast cancer with 90.4% receiving radiotherapy (RT) and 60.4% of patients receiving boost RT (3 fractions of 3-Gy). RESULTS After a median follow-up of 58 months, five of the 563 (0.9%) patients developed IBTR. The 5-year actuarial IBTR rate was 1.1%. In terms of distant disease recurrence (DDR), 29 of the 563 (5.2%) had DDR during follow-up, giving a 5-year actuarial DDR rate of 5.4%. The 5-year breast cancer specific survival was 95%, with the poorer NPI groups having worse breast cancer specific survival (Log-rank, P<0.0001). More importantly, patients with IBTR had a shorter breast cancer-specific survival than those who were IBTR-free (Log-rank, P<0.0001). CONCLUSIONS Our treatment regimen, combining BCS with a 5-mm target margin and hypofractionated 40-Gy RT, results in an extremely low rate of IBTR, and compares favourably with the target IBTR rate of <5% defined by the Association of Breast Surgeons (ABS) at BASO guidelines.