BACKGROUND: Our study was undertaken to investigate the pathogenesis and possible risk factors for post-renal transplantation diabetes mellitus (PTDM). METHODS: we recruited 114 patients with normal glucose tolerance, and performed the 75 g oral glucose tolerance tests (OGTT) and the short insulin tolerance tests 1 week before and 9~12 months after transplantation, respectively. RESULTS: The subjects were classified into three groups on the basis of OGTT after transplantation by WHO criteria: 1) 36 (31.6%) subjects with normal glucose tolerance; 2) 51 (45.7%) subjects with impaired glucose tolerance; and 3) 27 (23.7%) subjects with post-renal transplantation diabetes mellitus. Dosages of steroid and cyclosporin-A (CsA) were equivalent among the 3 groups. Before transplantation, the fasting and 2-h plasma glucose, and proinsulin/insulin (PI/I) ratios were significantly higher in the IGT and PTDM groups than in the NGT group, but insulin sensitivity index (ISI) was not different among 3 groups. In addition, the area under the curve (AUC)-insulin on OGTT was significantly lower in the PTDM group than in the NGT group. After transplantation, however, ISI was increased in all groups. Furthermore, the ISI and PI/I ratios revealed significantly higher values in the PTDM group than in the NGT group after transplantation. CONCLUSION: These results revealed that fasting and 2-h plasma glucose levels, as well as proinsulin/insulin ratio before transplantation, which may all be indicators of beta-cell dysfunction, could be the predictors for the development of PTDM and beta-cell dysfunction rather than insulin resistance was proved to be the main factor for the pathogenesis of PTDM.