OBJECTIVE To investigate the outcomes in patients who have pre-existing diabetes and those who develop post-transplant diabetes mellitus (PTDM). METHODS We retrospectively reviewed the charts of 939 patients who received a first functioning renal transplant in the cyclosporine (CyA) era between 1984 and 1999. RESULTS Sixty-six (7%) patients had renal failure due to insulin-dependent diabetes mellitus (IDDM) and 7 (0.8%) patients due to non-insulin-dependent diabetes mellitus (NIDDM). Ten (1.1%) patients had coexistent diabetes and 48 (5.1%) recipients developed PTDM. The mean graft survival for the patients with PTDM was 9.7 yr versus 11.3 yr for the non-diabetic patients, while mean graft survival was 10.1 yr for patients with IDDM and 2.9 yr with NIDDM and 8.3 yr for those with coexistent diabetes (p=ns). However, there was a statistically significant difference in patient survival between patients who developed PTDM and in those who did not develop this complication. The mean survivals of patients with IDDM, NIDDM, coexistent diabetics and PTDM were 8.4, 3.7, 8.6 and 10.3 yr, respectively. The mean survival of the patients without pre-existing diabetes or PTDM was 12.8 yr (p<0.001). The survival of patients older than 55 yr with PTDM was no different to the control group. However, in those younger than 55 yr, PTDM was associated with a higher risk of death (relative risk of 2.54, p<0.001). Fifty percent of patients with IDDM developed acute rejection episodes, whereas rejection rate was 57.1% in NIDDM group, 50.0% in the PTDM group, 20.0% in the coexistent diabetes group and 44.3% in the control group (p=ns). CONCLUSIONS Patient survival, but not graft survival, was adversely affected by both pre-existing diabetes and by PTDM, particularly in those with an age less than 55 yr.