Presently, there is no consensus on whether synchronous breast cancer has the same disease origin as the primary tumor, or if it is a completely independent second primary. This study explores this concept in both synchronous and metachronous breast cancer looking specifically at their receptor characteristics and level of differentiation. A retrospective chart analysis of 114 patients with synchronous or metachronous breast cancer treated at a single institution between January 1991 and March 2004 was done. Sixty-three per cent of the patients were diagnosed with metachronous breast cancer. Synchronous breast cancer was histologically more aggressive (P < 0.05) with a significantly higher number of patients having poorly differentiated tumors, a greater number of metastases involving a larger number of organs (P < 0.05), and lower average survival compared with the metachronous group (P < 0.005). Both the first and second tumor in both groups were similar in hormone receptor status, histologic subtype, and grade. Synchronous breast cancer is more aggressive and has a poorer outcome than metachronous breast cancer. Concordance in hormone receptor status, grade, and histologic subtype between different tumors within the same patient suggests, but does not completely support, a monoclonal origin. Analysis applied here is crude and more specific methods of analysis such as DNA microarray would be required to infer such a conclusion.