Can internal mammary chain treatment decrease the risk of death for patients with medial breast cancers and positive axillary lymph nodes? The effect of internal mammary chain treatment on each type of malignant death-related event was analyzed in 1195 patients with operable breast cancer and histologically involved axillary lymph nodes. A group of 135 patients who had no internal mammary chain treatment was compared with a control group of 1060 patients who were treated by surgery and/or postoperative radiation therapy. In a multivariate analysis taking into account age, clinical size of the tumor, histoprognostic grading, and the number of positive axillary lymph nodes, quantitative interaction tests were used to determine whether the effects of internal mammary chain treatment on each type of malignant event were significantly different for patients with a lateral tumor compared with those with a medial tumor. The authors found that the effects of this treatment on the risks of distant metastases and of secondary breast cancer were not the same for the patients with a medial tumor as for those with a lateral tumor. For the untreated patients with a medial tumor, the risks of distant metastases and second breast cancer were, respectively, 1.6 (P = 0.02) and 2.9 (P = 0.02), compared with the treated patients. Conversely, for women with lateral tumor, no difference between the two treatment groups was observed. Thus, internal mammary chain treatment may improve long-term survival rate in patients with a medial tumor and positive axillary lymph nodes essentially by decreasing the risk of development of distant metastases (mainly brain, distant lymph nodes, multiple simultaneous metastases) and/or a secondary breast cancer. 