In a previous study of 131 breast carcinomas with medullary features, we evaluated the diagnostic inter- and intraobserver variation and its prognostic implications using the criteria of typical (TMC) and atypical (AMC) medullary carcinoma of the breast put forward by Ridolfi et al. (1977). We found a considerable interobserver variation as well as intraobserver variation, with significant implication on prognosis, and concluded that the histopathological definition of MC must be sharpened and simplified in order to increase the diagnostic reproducibility. In the present study of the same population of 131 patients with breast carcinomas with medullary features we have examined inter- and intraobserver variation concerning 11 histopathological characteristics. Furthermore, we have analysed the prognostic importance of these 11 histopathological features, and the prognostic implications of the observed inter- and intraobserver variation. Based on the observations, we have eliminated criteria with poor inter-/intraobserver agreement as well as those implying no or minimal impact on the prognosis. We propose a new simplified histopathological definition of medullary carcinoma of the breast (MC), retaining reproducible, prognostically significant criteria (syncytial growth pattern and diffuse, moderate or marked mononuclear infiltration). The prognosis of MC, based on this definition, is significantly better than those of infiltrating ductal carcinomas grade II + III.