The purpose of this study was to address the question whether patients with bilateral breast cancer (BBC) have a worse prognosis in terms of recurrence and survival than patients with primarily unilateral breast cancer (UBC) following breast-conserving treatment (BCT). From 1983 to 2000, a total of 1760 BCT were registered in the Radiotherapy Department of the Medisch Spectrum Twente. We defined synchronous a BBC as cancer diagnosed in both breasts at the same time or within a period of 3 months of diagnosis of the first tumor. One thousand seven hundred and sixty BCT were performed on 1705 patients, 26 of whom presented with BBC. Of these 26 patients, 18 had BCT for both breasts. A higher proportion of patients with BBC showed more tubular carcinoma (P=0.029) and medially located tumors (P=0.076) than those with UBC did. The 5- and 10-year local recurrence rates (LRRs) were 4.5% and 9.1%, respectively, in BBC patients, as against 3.3% and 7.6% for UBC after BCT. The 5- and 10-year distant metastasis rates were 26.9% and 50.7%, respectively, for BBC as against 13.4% and 21.1% for UBC after BCT (P=0.065 and P=0.014, respectively). The 5- and 10-year disease-specific survival (DSS) rates for the 1705 patients were 82.1% and 41%, respectively, after BBC, and 91.4% and 84% after UBC (P=0.086 and P=0.0045, respectively). Patients with BBC have a higher rate of distant metastasis and a worse DSS than those with UBC. As the LRR is similar for BBC and UBC, BCT is not contraindicated in BBC. The incidence of BBC is low, at 1.5% which makes it difficult to reach any more definitive conclusions on outcome and treatment.