AIMS: To determine the effect of delay in postoperative radiotherapy on local recurrence and overall survival in women receiving partial mastectomy for breast cancer. MATERIALS AND METHODS: This was a systematic review and meta-analysis of published literature. Relevant reports were identified from MEDLINE, EMBASE and the Cochrane Register of Controlled Trials in all languages from 1975 to April 2015, in addition to the abstracts from the annual meetings of major radiotherapy conferences from 2000 to 2011. Reference lists were hand searched to find additional relevant reports and OvidSP's 'Find Citing' function was used to find studies citing papers identified in the primary search. Studies were included if they met the following criteria: (i) all patients received partial mastectomy and radiotherapy, (ii) a delay from surgery to radiotherapy was reported and (iii) one or more of local control/failure and/or survival were reported. Observational studies and randomised controlled trials were included. Studies including patients with in situ disease were excluded. Studies were classified as high quality if they adequately controlled for factors known to be associated with the outcomes of interest. Study quality was independently assessed by three authors. Initial disagreements about three studies were resolved by consensus. Only high-quality studies were included in the primary analysis. Delay was modelled as a continuous variable and the relative risk of local recurrence and the relative risk of death are reported per month of delay. The study results were combined using a fixed-effects model. RESULTS: Thirty-four relevant publications including 79 616 patients were identified in the systematic review. Ten high-quality publications reported on local recurrence (13 291 patients) and four high-quality studies reported on overall survival (2207 patients). The relative risk of local recurrence per month of delay was 1.08 (95% confidence interval 1.02-1.14). The relative risk of death per month of delay was 0.99 (95% confidence interval 0.94-1.05). CONCLUSIONS: Delays in post-lumpectomy radiotherapy are associated with a significant increase in the risk of local recurrence. We recommend that waiting times for radiotherapy should be kept as short as reasonably achievable.