The risk of an internal mammary lymph node (IMN) metastasis and its prognostic value for patients with invasive breast cancer were assessed by evaluating 142 patients who had either a mastectomy with lymph node dissection or a biopsy of the IMN. By univariate analysis, overall survival significantly correlated with the patient's age, clinical axillary node status, tumor size, and DNA ploidy, as well as histologically confirmed axillary and IMN metastases. By multivariate analysis, however, only the presence of axillary and IMN metastases appeared to be an important independent factor affecting survival. However, the incidence of IMN metastases was associated significantly with age, clinical tumor and axillary node status, tumor size, axillary lymph node metastases, and DNA ploidy. Accordingly, the patient's age, tumor size, DNA ploidy, and axillary lymph node metastases proved to be effective variable for discrimination. Consequently, in predicting the presence of IMN metastases, a diagnostic accuracy of 82%, a sensitivity of 84%, and a specificity of 82% can be achieved by a discriminant function. We conclude that the discriminant function with these four variables is effective in assessing the risk of IMN metastases.