Whilst HLA matching is routine in renal transplantation the possible benefits of matching donor to recipient age have not been previously explored. The simultaneous effect on graft survival of donor and recipient age was therefore investigated for 274 consecutive first cadaver transplant recipients treated by cyclosporin immunosuppression in two centres. The overall graft survival was 77%, and was not significantly different between the two centres. Individually there was no significant effect of donor or recipient age but taken together, the difference in age significantly affected graft survival (P less than 0.01) regardless of the mode of failure. The 1-year graft survival for all failures was 66.2% when the donor was 5 or more years older. 84.5% when the donor was 5 or more years younger and 71.7% when the donor was within 5 years of the recipient's age. Multivariate analysis, taking into account other variables (HLA matching, dialysis time and type, donor/recipient sex, local/imported kidneys, sensitivity, operation time, total ischaemic time, pre-operative transfusions) indicated that age difference was the single most important variable (P less than 0.01). The only other important covariate risk factor in improving graft survival was HLA-DR matching (P less than 0.05). Donor-recipient age difference is a potentially important recipient selection criterion in cyclosporin-treated renal transplant patients.