BACKGROUND: Obstructive Sleep Apnea Syndrome (OSAS) is a risk factor for resistant hypertension. Overnight polysomnography and portable sleep monitors are not generally available and questionnaires may be useful for screening OSAS. In a case-control study, we investigate the association between resistant hypertension and sleep disorders evaluated by the Berlin Questionnaire and Epworth Sleepiness Scale (ESS). METHODS: Cases were 63 patients with resistant hypertension (either systolic blood pressure (BP) > or =140 mm Hg or diastolic BP > or =90 mm Hg using at least three BP-lowering drugs, including a diuretic). Controls were 63 patients with controlled BP under drug treatment. All the patients were submitted to ambulatory BP monitoring and level III polysomnography by means of a portable monitor. The prevalence of high risk in the Berlin Questionnaire and high score in the ESS (>10) was compared between the groups. Diagnostic performance for OSAS of both questionnaires was calculated. RESULTS: The prevalence of high score in ESS was 44% in both groups. There was a higher prevalence of high risk for OSAS in Berlin Questionnaire in the cases group (78% vs. 48%; P < 0.001). In a logistic regression model, high risk for OSAS in Berlin Questionnaire was independently associated with resistant hypertension (odds ratio 4.1; confidence interval 95% 1.80-9.31; P < 0.01). Sensitivity and specificity for the diagnosis of OSAS was 85.5% (75.3-92.0%) and 65.0% (52.0-76.0%), respectively. CONCLUSIONS: High risk for OSAS assessed by the Berlin Questionnaire is highly prevalent and associated with resistant hypertension. This questionnaire may be used as screening for OSAS in patients with this clinical condition.