Earlier studies on breast cancer have shown the strong prognostic value of morphometric parameters (especially the morphometric prognostic index [MPI]) in comparison with clinical and classical pathologic parameters. It remained to be proven whether the prognostic value of the MPI holds for the subgroup of premenopausal patients. We have therefore investigated the value of different prognosticators in a group of 211 premenopausal breast cancer patients with long-term follow-up, 121 cases being lymph node-negative and 90 cases being lymph node-positive. The MPI, a multivariate combination of the mitotic activity index (MAI), lymph node status, and tumor size, was the best combined prognosticator (P less than .0001), exceeding the prognostic value of MAI, lymph node status, and tumor size as individual parameters and as indicators of histologic grade. Of all the features studied, the MPI had the best prognostic value in the lymph node-negative patients, while the MAI and MPI had the best prognostic value in the lymph node-positive patients. Since the MPI has been shown to be reproducible in intra- and interlaboratory studies and can be assessed with standard equipment in routine histologic sections, it is an attractive indicator for selecting high-risk lymph node-negative patients for systemic adjuvant therapy trials.