Objective: Obstructive sleep apnea (OSA) and primary aldosteronism are common in subjects with resistant hypertension; it is unknown, however, if the two disorders are causally related. This study relates plasma aldosterone and renin levels to OSA severity in subjects with resistant hypertension, and in those with equally severe OSA but without resistant hypertension serving as control subjects. Methods: Seventy-one consecutive subjects referred to the University of Alabama at Birmingham (UAB) for resistant hypertension (BP uncontrolled on three medications) and 29 control subjects referred to UAB Sleep Disorders Center for suspected OSA were prospectively evaluated by an early morning plasma aldosterone concentration (PAC) and renin level, and by overnight, attended polysomnography. Results: OSA (apnea-hypopnea index [AHI] ≥ 5/h) was present in 85% of subjects with resistant hypertension. In these subjects, PAC correlated with AHI (ρ = 0.44, p = 0.0002) but not renin concentration. Median PAC was significantly lower in control subjects compared to subjects with resistant hypertension (5.5 ng/dL vs 11.0 ng/dL, p < 0.05) and not related to AHI. In male subjects compared to female subjects with resistant hypertension, OSA was more common (90% vs 77%) and more severe (median AHI, 20.8/h vs 10.8/h; p = 0.01), and median PAC was significantly higher (12.0 ng/dL vs 8.8 ng/dL, p = 0.006). Conclusion: OSA is extremely common in subjects with resistant hypertension. A significant correlation between PAC and OSA severity is observed in subjects with resistant hypertension but not in control subjects. While cause and effect cannot be inferred, the data suggest that aldosterone excess may contribute to OSA severity.