The primary tumors of 106 female patients with breast cancer (with a mean follow-up of 17 years) were analysed for prognostic factors, with special emphasis on improved prognostic prediction of the small axillary lymph node-negative tumours. In addition to classic prognostic variables (histological type, nuclear grade, tumor size, node involvement), six morphometrically determined nuclear variables (mean nuclear area, SD of nuclear area, mean area of the 10 largest nuclei, maximum nuclear diameter, shortest nuclear diameter, mean nuclear perimeter, and SD of nuclear perimeter) and two mitotic indexes (mitotic activity index (MAI) and volume corrected mitotic index (M/V-index] were measured and related to the patient survival data. Mitotic indexes (p less than 0.001) as well as nuclear morphometric features (p less than 0.001) accurately predicted axillary lymph node involvement at operation. The axillary metastases that developed during the follow-up were also significantly related to mitotic indexes (p less than 0.001). The same indexes also predicted the recurrence free survival (p less than 0.001). The best predictors of cancer related survival were axillary node status and the mitotic indexes at the time of diagnosis (p less than 0.001). The potential of the M/V index in predicting the patient survival was equal to that of the axillary lymph node status. Of the two mitotic indexes, the M/V index was superior to the MAI in survival analysis. The results had us to advocate the inclusion of the newly introduced M/V index in the prognostic factors used in predicting the biological behaviour of breast cancer.