Common as well as unusual, heretofore unmentioned histopathologic features observed in 336 typical and 273 atypical medullary breast cancers from 6404 patients enrolled in various stage I and II protocols of the National Surgical Adjuvant Breast and Bowel Projects (NSABP) are presented. Both medullary types exhibited comparable pathologic findings, except for the infiltrative border and/or slight or absent tumor lymphoid infiltrate which by definition characterize the atypical form. Both also demonstrated a similar, high proclivity to be aneuploid, and to lack estrogen and progesterone receptors and nodal metastases. After appropriate statistical adjustments, survival (analyzed for 198 patients with typical and 149 with atypical medullary cancers) was found to be better for untreated, node-negative and node-positive patients treated with L-PAM + 5Fu who had typical medullary cancers than those with the NOS histologic type. The magnitude of this difference was 6% at 5 and 17% at 10 years post-operatively (cumulative odds = 1.81 with a 95% confidence interval of 1.08 - 3.3) for the former group, and 4% at 5 and 16% at 10 years (cumulative odds = 1.56 with a 95% confidence interval of 1.08 - 2.23) for the latter. Survival was comparable for patients with atypical medullary and NOS types in both situations. No clear difference in survival was found in untreated, positive node patients with the 3 histologic types examined, although the sample sizes in this subset were relatively small. This information as well as other pertinent considerations indicate that the prognosis of typical medullary cancer is not as 'good' as previously perceived. It is also concluded that there is insufficient evidence at present to exclude the atypical medullary variant as a histologic type of breast cancer.