In an earlier study of 235 breast cancers with medullary features, we concluded from a multivariate Cox regression analysis that only four histopathological features contained significantly positive prognostic information. In the present study, continuing our work on the same population base, we used these histological characteristics (predominantly syncytial growth pattern, no tubular component, diffuse stromal infiltration with mononuclear cells and sparse necrosis (< 25%), as diagnostic criteria for medullary carcinoma of the breast (MC). We found a significantly better prognosis for patients with MC than those with non-medullary carcinoma (NMC) or infiltrating ductal carcinoma (IDC). All tumours in the MC group were grade II or III (96% grade III). A significantly different distribution of general risk factors such as lymph node status, invasion, steroid receptor status, and menopausal status, was found between the group of MC and the control group of IDC grades II + III. Further, general risk factors, which are found to be of major prognostic importance in IDC, had little prognostic impact in MC. We found MC to be biologically unique, and patients with MC have a better than average prognosis compared to that of IDC. We propose a new histological definition of MC, but stress that prospective studies have to be performed.