The aim of this study was to assess the applicability of histopathological grading according to the protocol of Elston/Ellis and the Nottingham Prognostic Index (NPI) to a defined breast cancer population. The NPI is the sum of the individual scores concerning grade, tumour size, and lymph node status, each weighted according to regression coefficients of a Cox proportional hazard analysis and calculated for each individual breast cancer patient. 630 consecutive patients with invasive breast cancer diagnosed 1988-91 were retrospectively followed up and their tumours reviewed and graded. A Cox proportional hazard analysis was performed. Grade, lymph node status, and tumour size were statistically significant predictors of survival within the follow up period (median 7.2 years). Similar to NPI, a temporary index (Kalmar Prognostic Index, KPI) was derived and normalised to NPI for comparison (KPI(norm)). NPI and KPI(norm) gave similar prognostic power in spite of the differences of the patient populations from which the 2 indices were derived. Patients with NPI 4 or less had 0.66% breast cancer specific mortality during the follow up time. 14% of the patients with NPI 4.1-5 and 32% of those with an index sum 5.1-6 died from breast cancer during this time. Younger patients tended to have higher grade tumours. We advocate the common use of grade and the NPI in order to increase the comparability of groups of patients receiving different therapies.