The hallmarks of diagnosis of medullary breast cancer (MedBC) used by the authors since 1977 have been that the tumor is well circumscribed, has syncytial architecture in greater than 75% of its surface, contains diffuse inflammatory infiltrate, has atypical nuclei, and forms no glandular pattern. In order to assess the clinical utility of these criteria, we studied a series of 95 previously untreated, surgically operable patients with breast carcinoma at the Institut Gustave-Roussy (IGR) between 1960 and 1979. A diagnosis of MedBC was initially made for these patients or suspected based on abundant inflammatory stroma observed in a histologic evaluation. Using these criteria, 26 cases were identified as typical medullary carcinoma (TMC), 23 cases as atypical medullary carcinoma (AMC), and 46 cases as nonmedullary carcinoma (NMC). The 26 cases of TMC represent a very small fraction of the total infiltrating operable breast carcinomas diagnosed at IGR during the same time period. The prognosis for these 26 patients was much more favorable than for the other groups. They had a 10-year disease-free survival of 92% compared with 53% for the AMC group and 51% for the NMC group. Neither distant metastasis nor secondary primaries of the same histology were seen. Therefore, it is possible with the use of strict histologic criteria to distinguish a group of patients with a much more favorable prognosis. This histologic diagnosis alone renders a most favorable prognosis for the patient even if other factors such as large tumor size and lymph node involvement are present and, by inference, the only therapy needed is the removal of all tumor. In contrast, atypical forms have a prognosis no different from other atypical types of breast carcinomas without inflammatory stroma, and adjuvant therapy appears to be justified if other factors warrant it.