A series of 688 women with breast cancer were followed-up for a mean of 13 years. Tumour size, axillary lymph node status, histological grade, histological type and two mitotic indexes (M/V; MAI) were assessed and related to disease outcome. Primary tumour size (P less than 0.0001), the volume-corrected mitotic index (M/V) (P less than 0.0001), the mitotic activity index (MAI) (P = 0.0001), and histological grade (P = 0.0074) predicted axillary lymph node status. Recurrence as well as recurrence-free survival was significantly related to the axillary lymph node status (P less than 0.0001), M/V index (P less than 0.0001), MAI (P less than 0.0001), tumour size (P = 0.0031) and histological grade (P = 0.0208). Multivariate analyses disclosed the tumour size and M/V index as independent predictors of axillary metastasis at diagnosis. Recurrence was related independently to M/V index, axillary metastasis and tumour size. Independent predictors of recurrence-free survival in Cox's analysis were M/V index and axillary lymph node status. Axillary lymph node status (P less than 0.0001), tumour size (P less than 0.0001), M/V index (P less than 0.0001), MAI (P less than 0.0001) and histological grade (P = 0.0009) predicted survival in that order. Cox's analysis showed that axillary lymph node status was the most important independent predictor of survival followed by tumour size and M/V index. In a separate Cox's analysis of axillary-lymph-node-negative patients the M/V index and tumour size were independently related to survival. In conclusion the M/V index is an important prognostic factor in breast cancer and also in axillary-lymph-node-negative breast tumours.