Intracellular activities of sodium and calcium were determined in red cells of patients with obesity. Compared to normal people mean intracellular sodium and calcium were higher in obese patients. However, increased intracellular sodium and calcium could only be observed in those patients with obesity suffering from hypertension or showing a familial disposition to hypertension. In contrast there was no difference in intracellular sodium and calcium between obese normotensives lacking a familial disposition to hypertension and normal people. Thus, our results suggest, that the observed variations in intracellular sodium and calcium in obesity are due to an enhanced blood pressure or a familial disposition to hypertension and not specific for obesity.