The adequate timing of adjuvant radiotherapy (RT) in breast cancer has become a subject of increasing interest in recent years. A population-based study was undertaken to determine the influence of demographic and clinical factors on the postoperative RT delay in patients treated with breast-conserving surgery (BCS) and to assess the impact of delay on survival. In total, 7800 breast cancer patients treated with BCS and adjuvant RT between 1986 and 1998 in Yorkshire were included in the study. The median interval between surgery and the start of RT (S-RT interval) was 8 weeks (7 weeks for chemotherapy negative and 11 for chemotherapy positive patients). This interval increased substantially over time from 5 weeks during 1986-1988, irrespective of patients' chemotherapy status, to 10 and 17 weeks among chemotherapy negative and chemotherapy positive patients, respectively, in 1997-1998. The S-RT interval was also significantly influenced by travel time to RT centre, year and at which RT centre patient had the treatment (P<0.001). Overall, 5-year survival was 82%. Patients with S-RT intervals longer than 9 weeks had a trend towards an increased relative risk of death. This reached a statistical significance at 20-26 weeks (RR 1.49, 95% CI (1.16-1.92)). The findings of our study suggest that delaying the initiation of RT for 20-26 weeks after surgery is associated with decreased survival in patients treated with conservation surgery.