The objectives of this analysis were (1) to assess whether low serum adiponectin concentrations are associated with insulin resistance, metabolic syndrome, and new-onset posttransplantation diabetes mellitus (PTDM) and (2) to examine the possible effects of immunosuppressive and antihypertensive therapies on circulating adiponectin levels after renal transplantation. A total of 172 consecutive previously nondiabetic renal transplant recipients were examined 3 mo after transplantation, the majority (n = 167) with an oral glucose tolerance test. Serum adiponectin was measured by an in-house time-resolved immunofluorometric assay. Insulin secretion and insulin sensitivity were estimated by previously validated oral glucose tolerance test-derived indexes. One- and 6-yr follow-up data were available in subgroups of patients. Lower adiponectin levels were significantly associated with insulin resistance but not with insulinopenia. Patients with low adiponectin levels (first quartile) had significantly higher odds of PTDM (odds ratio [OR] 3.6; 95% confidence interval [CI] 1.1 to 12.7; P = 0.049) and metabolic syndrome (OR 3.9; 95% CI 1.6 to 9.5; P = 0.003) than patients in the upper (fourth) quartile. The increased risk for PTDM in patients with low adiponectin levels remained significant after adjustment for age, steroid dose, and family history of diabetes. Treatment with beta blockers was independently associated with lower serum adiponectin levels, and total steroid dose was associated with higher serum adiponectin levels. Low baseline adiponectin levels were also associated with significantly higher odds of PTDM at 6 yr (OR 6.9; 95% CI = 1.1 to 41.8; P = 0.037). Serum adiponectin levels correlate with posttransplantation insulin sensitivity and glucose tolerance. Glucocorticoids and beta blockers seem to have opposite effects on circulating adiponectin levels.