The obstructive sleep apnea syndrome is typically associated with conditions known to increase insulin resistance as hypertension, obesity, diabetes. We investigated, whether obstructive sleep apnea itself is an independent risk factor for increased insulin resistance and whether continuous positive airway pressure treatment improves insulin sensitivity. 40 patients (apnea-hypopnea-index > 20) were treated with continuous positive airway pressure. Before, 2 days after and after 3 months of effective continuous positive airway pressure treatment, hyperinsulinemic euglycemic clamp studies were performed. Insulin sensitivity significantly increased after 2 days (5.75 [plusmn] 4.20 baseline vs. 6.79 [plusmn] 4.91 [mu]mol/kg x min; p = 0.003) and remained stable after 3 months of treatment. The improvement in insulin sensitivity after 2 days was much greater in patients with a body mass index < 30 kg/m² than in more obese patients. The improved insulin sensitivity after two nights of treatment may reflect a decreasing sympathetic activity, indicating, that sleep apnea is an independent risk factor for increased insulin resistance. The effect of continuous positive airway pressure on insulin sensitivity is smaller in obese patients than in non-obese patients, suggesting that in obese individuals insulin sensitivity is mainly determined by obesity and to a smaller extent by sleep apnea.