Recent studies indicate that resistant hypertension (RHTN) is present in about 12% of the treated hypertensive population. However, patients with true RHTN (confirmed out of the office) have not been widely studied. We prospectively studied 204 patients (123 male, 81 female, mean age 48.4 years, range 19-65 years) with truly RHTN (ambulatory daytime mean blood pressure >135/85 mm Hg). We evaluated the frequency of obstructive sleep apnea (OSA), renal artery stenosis (RAS), primary aldosteronism (PA) and other secondary forms of hypertension (HTN) and conditions. Mild, moderate and severe OSA were present in 55 (27.0%), 38 (18.6%) and 54 (26.5%) patients, respectively. Secondary forms of HTN were diagnosed in 49 patients (24.0%), the most frequent being PA (15.7%) and RAS (5.4%). Metabolic syndrome (MS) was present in 65.7% of patients. Excessive sodium excretion was evident in 33.3% of patients and depression in 36.8% patients. In patients with RHTN, OSA and MS were the most frequent conditions, frequently overlapping with each other and also with PA. Our data indicate that in the vast majority of patients with truly RHTN, at least one of three co-morbidities-OSA, MS and PA-is present. Other conditions, even though less frequent, should also be taken into the consideration.