Background Breast-conserving surgery (bcs) and radiation therapy (rt) are the standard of care for early breast cancer, although some women receive ipsilateral mastectomy or adjuvant tamoxifen, both of which can be appropriate alternatives to rt. Objectives of the present study were to determine the proportion of women who are treated appropriately after bcs and to identify factors associated with non-receipt of rt. Methods This retrospective cohort study used Ontario data linked at the Institute for Clinical and Evaluative Sciences to examine 33,718 patients who received bcs during 2004–2010. Primary outcome was rt receipt. The ipsilateral mastectomy rate and patient, surgeon, and setting variables were measured. Results Of the study patients, 86.1% received either rt or completion mastectomy; in the cohort less than 70 years of age, 90.8% received rt or completion mastectomy. Among patients less than 70 years of age, 3 risk factors for nonreceipt of rt were identified: age less than 46 years, treatment in a non-academic institution, and earlier year of initial bcs. Additionally, in the overall cohort, rt non-receipt was associated with high comorbidity, more than 40 km to the cancer centre, income quintile, and breast care specialization. Conclusions In Ontario, 90.8% of patients less than 70 years of age are appropriately treated for early breast cancer; approximately 1 in 10 do not receive rt or completion mastectomy. Based on those findings, women less than 46 years of age might be at increased risk of recurrence and death because of incomplete treatment. It also appears that academic centres more effectively treat breast cancer; however, breast cancer care appears to be improving over time in Ontario.