The mechanisms underlying the adverse cardiovascular effects of increased salt intake are incompletely understood, but parallel increases in serum sodium concentration may be of importance. The aim of this retrospective cohort study was to investigate the relationship between serum sodium, hypertension and incident cardiovascular disease (CVD). Routinely collected primary care data from the Royal College of General Practitioners Research and Surveillance Centre were analysed. A total of 231,545 individuals with a measurement of serum sodium concentration at baseline were included. Exclusion criteria were: age < 40 years; abnormal serum sodium; diabetes mellitus; prior CVD event; stage 5 chronic kidney disease; and liver cirrhosis. The primary outcome was incident CVD (myocardial infarction, acute coronary syndrome, coronary revascularisation, stroke, transient ischaemic attack or new heart failure diagnosis) over 5 years. There was a ‘J-shaped’ relationship between serum sodium concentration and primary cardiovascular events that was independent of established risk factors, medications and other serum electrolytes. The lowest cardiovascular risk was found with a serum sodium between 141 and 143 mmol/l. Higher serum sodium was associated with increased risk in hypertensive individuals, whereas lower concentrations were associated with increased risk in all individuals. Therefore, alterations in serum sodium concentration may be a useful indicator of CVD risk. Higher serum sodium could have a direct effect on the vasculature, particularly in hypertensive individuals. Lower serum sodium may be a reflection of complex volume and neuroendocrine changes.