We evaluated the relationship between blood pressure and serum sodium level in morbidly obese patients. Forty patients had at least 2-3 blood pressure measurements and two measurements of serum sodium. The correlation between serum sodium and systolic blood pressure was 0.66 (p < 0.01). The correlation between diastolic pressure and serum sodium was 0.40 (p < 0.025). Our observation linking serum sodium to blood pressure provides a lead to study mechanisms common to regulation of blood sodium and blood pressure in morbid obesity.