Several studies have demonstrated a potential for a bi-directional relationship between obstructive sleep apnoea (OSA) and primary aldosteronism (PA), however many of these studies are limited to subsets of patients diagnosed with hypertension or existing PA. We recruited 85 patients who were attending a diagnostic sleep study, otherwise unselected, for suspected OSA and screened for PA measuring serum aldosterone, direct renin concentration and aldosterone:renin ratio (ARR). OSA was diagnosed via diagnostic overnight polysomnography and neck and calf circumferences were measured before and after polysomnography. Blood pressure was measured using a 24-hour ambulatory blood pressure monitor (ABPM) in 58 participants. 2 out of 85 participants were identified to have likely PA (2.4%) based on elevated ARR or suppressed renin on blood test results despite being on anti-hypertensives known to elevate renin, together with CT evidence of an adrenal adenoma. Another 10 were identified to have possible PA (11.8%) due to evidence of suppressed renin while on interfering anti-hypertensive medications. In participants with both OSA and hypertension (n=41), the prevalence of likely or possible PA increases to 29.3%. Differences in the awake diastolic blood pressure load were significant across ARR tertiles (p=0.035). No correlation was observed between aldosterone, renin or ARR and the apnoea-hypopnea index (AHI) using multiple regression analysis despite adjusting for interfering medications and hypertension status. In conclusion, we were unable to confirm a relationship between OSA and PA but found that the potential prevalence of PA amongst hypertensives with OSA is 29.3%. Our study results suggest that a diagnostic sleep study may be a good opportunity to screen for PA, given that hypertension is common in both OSA and PA.