Purpose: The aim of this study was to determine the effect of timing of radiotherapy after conservative breast surgery on local recurrence in women with positive resection margins and young age, treated without systemic therapy. Methods and materials: A total of 568 patients with T1 and T2, N0 breast cancer were treated with breast-conserving surgery and breast irradiation, between January 1, 1985, and December 31, 1992, at the London Regional Cancer Centre. 63 patients (11.1%) had positive/close resection margins (< 2 mm) and 48 patients (8.4%) were age < or = 40 years. For patients with positive resection margins, the time intervals from breast surgery to breast irradiation used for analysis were, 0 to 8 weeks, > 8 to 12 weeks and > 12 weeks. For patients < or = 40 years, the intervals used for analysis were 0 to 8 weeks and > 8 weeks. Results: Median follow up was 11.2 years. For patients < or = 40 years, local recurrence rate at 5 and 10 years was 17.2% and 19.8% respectively. Four patients (17.4%) treated in the 0-week to 8-week interval and 7 patients (28.0%) treated in the > 8 week interval had local recurrences. For patients < or = 40 years with positive resection margins, the local recurrence rate was 25.0%. For patients with positive resection margins, 5-year and 10-year local recurrence rates were as follows: 0 to 8 weeks, 0% and 10.5% respectively; > 8 to 12 weeks, 10.3% and 10.3% respectively; and > 12 weeks, 13.3% and 20.0% respectively. Conclusion: Patients < or = 40 years have an increased local recurrence rate which occurs early. Patients with positive resection margins have higher local recurrence rates that become apparent when breast irradiation is delayed.