New reports suggest a high prevalence of primary aldosteronism (PA) in patients with essential hypertension, especially in those whose blood pressure (BP) is difficult to control. We recently demonstrated that obstructive sleep apnea (OSA) is highly prevalent in refractory hypertension (RH). In that study ambulatory sitting plasma aldosterone concentration (PAC) and plasma renin activity (PRA) were measured in patients taking BP-lowering drugs as a screening test for PA on a free-choice diet. None had definite PA, and no confirmatory tests were done. In this report we ascertained the relationship between apnea-hypopnea index (AHI), an indicator of OSA severity, and PAC, PRA and the PAC/PRA ratio (ARR). Moderate to severely hypertensive patients (sitting clinic BP of 155/90 on a mean of 3.1 antihypertensive drugs) were classified as having no or mild, moderate and severe OSA, based on an AHI of <15, 15–29, ≥30 respectively from overnight polysomnography. The 24 men and 22 women studied had a mean age of 57.4 y, body mass index (BMI) of 33.3 kg/m2 and AHI of 24.3 events/h. The frequency of PAC ≥15 ng/100mL was 10.9%, and ARR ≥25, 13.0%. Results, categorized by OSA severity and expressed as mean ± standard deviation, are presented in Table. Additionally, AHI was a significant predictor of ARR (p = 0.026). Taken together, these findings suggest that severe OSA is associated with a biochemical profile that can suggest PA. A more systematic study is now needed to explore these relationships.AHI, events/h (no. of pts)PAC, ng/100 mLPRA, ng/mL/hARR<15 (19)6.8 ± 4.266.9 ± 16.457.8 ± 7.2815–29 (16)7.0 ± 4.863.7 ± 7.6010.3 ± 9.82≥30 (11)12.1 ± 6.87*6.6 ± 12.2513.8 ± 11.90*p = 0.021 (unadjusted) or 0.037 (adjusted for BMI) in ANOVA. p = 0.021 (unadjusted) or 0.037 (adjusted for BMI) in ANOVA.