Abstract Background Hypertensive patients with primary aldosteronism (PA) have a higher risk of cardiovascular complications than those with blood pressure-matched essential hypertension. The excess cardiovascular consequences of PA can be attributed to the pro-inflammatory effect of excessive aldosterone and mineralocorticoid receptor activation in a range of peripheral tissues and cell types. The neutrophil-to-lymphocyte ratio (NLR) is a widely available markers of inflammation which has been shown to predict cardiovascular outcome in the general population. This study aims to evaluate the use of NLR as a potential biomarker of PA and PA severity. Methods Patients with PA (n=355) were identified from two large PA databases in Australia and China, while controls (n=222) were patients with hypertension who were referred for assessment but did not meet the diagnostic criteria for PA. The NLR was retrospectively collected from routine full blood examination, prior to commencement of targeted treatment for PA. Results The NLR did not differ between PA patients and hypertensive controls (median 2.3 and 2.4, p=0.563). However, amongst patients with PA, the NLR was positively correlated with baseline and post-saline aldosterone levels (r=0.22 and p<0.001 for both) and negatively correlated with serum potassium (r=-0.15, p=0.006). Furthermore, in a logistic regression analysis of data from patients with PA, the NLR predicted the presence of co-morbid CKD (defined as eGFR < 60mL/min/1.73m 2) with an odds ratio of 1.5 (p=0.003). Conclusion Whilst the NLR did not distinguish PA from controls, it was a marker of PA severity, being associated with aldosterone concentration as well as the presence of CKD. A prospective study is needed to further clarify the role of NLR in predicting end-organ damage associated with PA.