We reviewed 30 cases of breast cancer originally diagnosed as medullary carcinoma during a 10-year period. Of the 30, only 9 proved to be "typical medullary carcinoma" with the associated favorable prognosis. Seven of the remaining 21 cancers were reclassified as "atypical medullary carcinoma," and the remaining 14 were nonmedullary cancers. Overdiagnosis of medullary carcinoma could lead to inappropriately conservative therapy, since medullary carcinoma is a prognostically favorable neoplasm. Those involved with diagnosis and treatment of patients with breast cancer should be aware of this potential problem.