Nucleolar organiser regions (NORs) were stained in paraffin-embedded biopsy specimens of 80 female breast carcinomas by the silver (Ag) technique. The patients were prospectively followed up for a mean of 12.4 years (range 11.5-13.3). The number of different types of Ag-NORs was correlated with the histological grade, clinical stage, DNA ploidy, S-phase fraction (SPF) and clinical outcome. Grade III tumours showed higher Ag-NOR counts than low grade tumours. The total number of Ag-NORs (P = 0.0059) and the number of dispersed Ag-NOR (P = 0.0199) were significantly related to DNA ploidy aneuploid tumours showing higher Ag-NOR counts. The number of aggregated Ag-NORs was predictive (P = 0.0413) for the development of metastatic disease during follow-up. On the other hand, crude, cancer-related or recurrence-free survival could not be predicted significantly by the Ag-NORs. The results suggest that the number of Ag-NORs is clearly related to the proliferative activity in breast cancer, but the prognostic value of Ag-NOR counting is inferior to the previously recognised prognostic factors.