To investigate the risk factors and clinical characteristics of postrenal transplant diabetes mellitus (PTDM), we reviewed the records of 177 renal allograft recipients in Maryknoll Hospiatal whose allografts had functioned longer than 6 months. Nineteen patients (10.7%) developed PTDM at 5.0+/-7.8 (1-52) months; 9 (47%) of these within 1 month. PTDM patients were older than nondiabetic renal transplants (42+/-2 vs 37+/-1 years, P<0.05). Body mass index tended to be higher in PTDM (23.5+/-1.0 vs 21.8+/-0.3kg/m2, P=0.09). Number of acute rejections (0.6+/-0.2 vs 0.5+/-0.1) and serum creatinine at 1 year after transplantation (1.2+/-0.8 vs 1.3+/-0.3mg/dL) were not different. Fasting (103.6+/-10.4 vs 84.4+/-1.6mg/dL, P<0.05) and postprandial (189.2+/-24.8 vs 118.6+/-2.3 mg/dL, P<0.01) blood sugars, measured before transplantation, were higher in PTDM. CsA blood level at 1 month posttransplantation was higher in PTDM (350+/-34 vs 279+/-8ng/mL, P<0.05). Fasting serum insulin was significantly higher (28.2+/-12.2 vs 7.3+/-2.0 microunit/dL, P<0.05) and serum C-peptide tended to be higher in PTDM patients compared with euglycemic renal recipients (6.3+/-1.6 vs 3.8+/-0.9ng/dL, P=0.08). All the PTDM patients were treated by either insulin or oral agent; 15 of 19 required no treatment after 4.7+/-6.9 months. In conclusion, prevalence of PTDM was 10.7%. PTDM patients were older. Body mass index was tended to be higher. Fasting and postprandial blood sugars, measured before transplantation, were higher in PTDM. Faslting serum insulin was higher and C-peptide tended to be higher in diabetics. These results suggested that increased insulin resistance plays a major role in the pathogenesis of PTDM.