The results of renal transplantation in patients with juvenile-onset diabetes mellitus were compared to those of a well-matched control group of non-diabetic patients. All transplantations were performed between 1977 and 1988. In the diabetic group hypertension (72 versus 41%), coronary artery disease (17 versus 0%), and peripheral vascular disease (19 versus 0%) had been significantly more frequent pretransplantation. Fewer diabetic patients had previously been treated with dialysis therapy (69 versus 97%). Graft function measured by creatinine clearance after 1 year follow-up, and incidence of proteinuria were not significantly different. The overall graft survival was significantly worse in the diabetic group compared to the control group: 42 versus 69% after 60 months and 21 versus 62% after 90 months. This was caused by a significantly worse patient survival in the diabetic group after 105 months: 28 versus 78% in the control group. The graft survival following exclusion of the patients who died with a functioning graft did not differ significantly between the groups after 60 and 90 months: 62 and 31% in the diabetic group and 69 and 62% in the control group. The existence of any vascular disease before transplantation, especially pre-existing peripheral vascular disease, had a significant effect on mortality in diabetic patients (P = 0.0003). After transplantation, diabetic patients had significantly more cerebrovascular accidents (23 versus 3%), peripheral vascular disease (31 versus 3%), and number of infections (1.9 versus 1.2). Retransplantation was carried out in each group to the same extent, with the same success rate.