We analysed 170 histological samples of invasive ductal breast cancer from years 1988-91 by computerized nuclear morphometry, to find objective and quantitative thresholds for nuclear grade. Based on Kaplan-Meier curves reflecting survival and recurrence of disease and univariate analysis by Cox's regression, optimal thresholds were determined for features related to nuclear size and size variation. In our material, with mean follow-up time of 5 years 9 months, the determined thresholds for nuclear profile area (32 microm2 and 47 microm2), nuclear diameter (6.4 microm and 7.4 microm) and mean shortest nuclear axis (4.8 microm and 6.4 microm) best separated the cases with favourable, intermediate and unfavourable course of disease. In this material from the era of mammography and adjuvant therapy, the mean shortest nuclear axis was found to be a significant prognostic factor, with a risk ratio (RR) exceeded only by that of tumour size (RRs 2.9- and 3.5-fold respectively). The results suggest that morphometric grading criteria can be developed for application in Bloom-Richardson grading and in the Nottingham Prognostic Index.