The increased risk of cardiovascular morbidity and mortality among patients with sleep-disordered breathing (SDB) has been linked to arterial hypertension and insulin resistance. However, an effective antihypertensive agent for patients with SDB has not been identified. We investigated the effect of the angiotensin II subtype 1 receptor blocker olmesartan in hypertensive patients with SDB. This prospective, one-arm pilot study included 25 male patients with untreated SDB (mean age, 52.7 ± 11.4 years). We measured blood pressure, oxygen desaturation index (ODI), cardiac function using echocardiography, and insulin resistance using the homeostasis model assessment (HOMA) before and after 12 weeks of olmesartan therapy (mean dose, 17.6 ± 4.4 mg/day). Olmesartan significantly decreased systolic blood pressure (151.4 ± 8.0 vs. 134.0 ± 7.4 mmHg; P < 0.001), diastolic blood pressure (93.4 ± 7.1 vs. 83.9 ± 6.3 mmHg; P < 0.001), and HOMA index (3.7 ± 2.9 vs. 2.8 ± 1.9; P = 0.012). Furthermore, left ventricular ejection fraction significantly increased at 12 weeks (68.1 ± 5.1 vs. 71.6 ± 5.4%; P = 0.009). However, body mass index (BMI) and degree of SDB did not change (BMI, 26.6 ± 4.0 vs. 26.6 ± 4.2 kg/m2, P = 0.129; 3% ODI, 29.5 ± 23.1 vs. 28.2 ± 21.0 events/h, P = 0.394). Olmesartan significantly reduced blood pressure and insulin resistance in hypertensive patients with SDB without changing BMI or SDB severity.