Purpose: To look at the optimum timing of radiotherapy in breast-conserving therapy (BCT) in relation to outcome in breast cancer. Methods: We analyzed 1473 BCT on 1446 breast cancer patients from our prospective cohort, stage I or II, node-negative, and without adjuvant systemic therapy. Timing was defined as time from lumpectomy till radiotherapy. Patients were categorized into three timing tertiles: 1-36 days, 37-53 days, and 54-112 days. Results: The 10-year local relapse-free survival rates did not show significant differences between the three groups. The 10-year Distant Metastasis-Free Survival (DMFS) was 78.9% for the first tertile, versus 86.1% (HR 0.6; P=0.009) for the second, and 90.7% (HR 0.3; P<0.001) for the third. The 10-year Disease-specific Survival (DSS) was 83.8% for the first tertile, versus 90.6% (HR 0.5; P=0.007) for the second, and 97.2% (HR 02; P<0.001) for the third. Also in multivariate Cox regression analysis the second (HR 0.6; P=0.053) and the third tertile (HR 0.3; P=0.002) had significantly better DSS. Conclusion: Timing of radiotherapy in BCT for breast cancer seems to be highly important in relation to survival. This study shows a 40-70% relative survival benefit with timing after 36 days.