The prognostic value of nucleolar organizer regions and ploidy status in patients with Dukes' C colorectal cancer was determined. In addition, nucleolar organizer regions and ploidy status were compared with the established prognostic indices age, sex, tumour location and degree of histological differentiation. Fifty-one patients (27 men, 24 women) aged 35-81 years (median 64.8 years) were studied. Five years after presentation, 16 patients were alive and well. Survivors had significantly lower nucleolar organizer region counts (median value and range) in primary tumours 12 (8-15) and lymph node metastases 11 (8-15) than non-survivors 17 (14-25) and 18 (13-25) respectively (P less than 0.05). Fewer survivors had aneuploid tumours than non-survivors: four survivors compared with 17 non-survivors. In a regression analysis model, nucleolar organizer regions were the most important individual variable for predicting survival (chi2 = 15, P less than 0.01) while ploidy values (chi2 = 6, P less than 0.2) were equivalent to histological differentiation.