Risk factors for local and distant recurrence after breast-conserving therapy and mastectomy were compared to define guidelines for the decision making between both treatments. The data of two randomized clinical trials for stage I and II breast cancer patients were pooled. The total number of patients in the study was 1,772, of whom 879 underwent breast conservation, and 893, modified radical mastectomy. Representative slides of the primary tumor were available for histopathologic review in 1,610 cases (91%). There were 79 patients with local recurrence after breast-conservation and 80 after mastectomy, the 10-year rates being 10% (95% confidence interval CI, 8% to 13%) and 9% (95% CI, 7% to 12%), respectively. Age no more than 35 years (compared with age >60: hazard ratio HR, 9.24; 95% CI, 3.74 to 22.81) and an extensive intraductal component (HR, 2.52; 95% CI, 1.26 to 5.00) were significantly associated with an increased risk of local recurrence after breast-conserving therapy. Vascular invasion was predictive of the risk of local recurrence, irrespective of the type of primary treatment (P <.01). Tumor size, nodal status, high histologic grade, and vascular invasion were all highly significant predictors of distant disease after breast-conserving therapy and mastectomy (P <.01). Age no more than 35 years and microscopic involvement of the excision margin were additional independent predictors of distant disease after breast-conserving therapy (P <.01). Age no more than 35 years and the presence of an extensive intraductal component are associated with an increased risk of local recurrence after breast-conserving therapy. Vascular invasion causes a higher risk of local recurrence after mastectomy as well as after breast-conserving therapy and should therefore not be used for deciding between the two treatments.