Background: To investigate the issue of timing of radiation therapy (RT) after lumpectomy in relation to recurrences and outcome. Methods: Analysis was done on 1107 breast-conserving therapies (BCT) with 1070 women, all without lymph node metastasis and without any adjuvant systemic therapy. Timing was defined as time from lumpectomy till RT. Patients were categorised into tertiles: <45 days, 45–56 days, and 57–112 days. Results: Local control did not show a difference between the tertiles. The distant metastasis-free survival as well as the disease-specific survival showed a decreased outcome starting the RT to early after the lumpectomy. Conclusion: The results of this cohort study further refines the hypothesis that timing of RT in BCT might have an impact on outcome. It suggests that a randomised trial in timing of RT in BCT seems necessary to give a definite answer.