Morphometric methods were applied to predict the clinical course of individual patients with breast cancer. Measurement of tumour diameter, assessment of mitotic and cellular indices, and quantitative microscopy of nuclear features were assessed together with nuclear features and histological grades. Of the tumours from 78 patients investigated, 42 had died from metastases within 6.5 years ('non-survivors'), while the other 36 were alive and well without evidence of metastases at the end of the follow-up period (minimum 6.5 years) ('survivors'). If the tumours of the 42 non-survivors are compared with those of 36 survivors, there are many reproducible significant differences, the most important being cellularity index and mitotic activity index, followed by quantitative microscopical nuclear parameters and nuclear and histological grade. Discriminant analysis, of the quantitative microscopical data alone showed 82% of all patients to be correctly classified as survivor or non-survivor. By contrast with the axillary lymph node invasion status alone, or the tumour diameter and axillary lymph node status together, 59% and 64% of the patients were predicted correctly as survivor or non-survivor. With a more realistic statistical approach of discriminant analysis, 78% of the patients were classified correctly with quantitative microscopy, in place of 54% with the axillary lymph node status, 56% with the TNM-system and 64% with a combination of TNM system and nuclear and histological grade. Morphometry thus seems possible to predict the outcome of individual patients more accurately than with the usual staging/grading methods. This technique might therefore prove to be useful in the selection of patients for adjuvant chemotherapy.