One hundred and ninety-five aged (mean age: 67+/-4.8 years), non-insulin dependent diabetic patients underwent a randomised single-blind study for investigating the effect of statin administration on insulin resistance and respiratory quotient. After 4 weeks run-in period, all patients were randomised in three groups: placebo (n=67), simvastatin (10 mg/day) (n=61) and atorvastatin (5 mg/day) (n=67). Each treatment period lasted 8 weeks. At the beginning, after the run-in and at the end of the study, insulin resistance was assessed by homeostasis model assessment (HOMA) index, while respiratory quotient (Rq) was evaluated by indirect calorimetry. Statins versus placebo significantly lowered plasma total, LDL-, HDL-cholesterol and triglyceride concentrations and improved insulin resistance and Rq and metabolic control. Atorvastatin had a greater effect than simvastatin on plasma triglyceride concentration (-26.3+/-3.1 vs. -19.7+/-2.8%, P<0.03), HOMA index (-13.1+/-0.6 vs. -9.1+/-0.9%, P<0.05), Rq (5.9+/-0.4 vs. 3.1+/-0.5%, P<0.05) and glycosylated haemoglobin (-11.2+/-0.3 vs. -7. 1+/-0.4%, P<0.05). In the whole group of subjects (n=195) and at the end of the study, changes in plasma triglyceride concentrations were significantly correlated with the change in the HOMA index (r=0.44, P<0.001) and age and BMI adjusted-Rq (r=-0.32, P<0.005). Multivariate analyses demonstrated that decline in plasma triglyceride concentration was a significant determinant for explaining the effect of statin on insulin resistance and Rq. In conclusion our study demonstrates that statin administration is useful for controlling dyslipidemia in NIDDM patients and for improving the metabolic control. With regard to this latter aim, atorvastatin seems to be more powerful than simvastatin.