Obstructive sleep apnea (OSA) is independently associated with glucose intolerance and insulin resistance, and recent studies have shown that continuous positive airway pressure (CPAP) improves insulin sensitivity. The objective of this study was to describe the change in glycosylated hemoglobin (HbA1c) after treatment with CPAP in patients with type 2 diabetes mellitus and OSA. To test this hypothesis, we performed a retrospective analysis of 38 patients seen in the sleep clinic of an urban public teaching hospital. All patients had OSA and type 2 diabetes mellitus, and their diabetic medication regimen had remained unchanged during the period of CPAP therapy. Sixty-one percent were men, body mass index was 42+/-9.5 kg/m(2), and the Apnea-Hypopnea Index was 53+/-36 per hour. HbA1c before therapy with CPAP was 7.8+/-1.4% and decreased to 7.3+/-1.3% after 134+/-119 days of therapy (p<0.001). Treatment with CPAP leads to a clinically significant drop in HbA1c in patients with type 2 diabetes mellitus and severe OSA.