Endolymphatic hydrops is the histopathologic hallmark of Meniere's disease. The cause of excess inner ear fluid is unknown, but the possibility of a systemic abnormality of fluid–solute regulation has been considered. One indicator of systemic fluid and electrolyte balance is the enzyme renin. Plasma renin levels were measured in patients with a clinical diagnosis of Meniere's disease and compared to a group of patients who did not have Meniere's disease. The mean plasma renin concentration of patients with Meniere's disease was statistically lower than that of the other subjects. The study's results are consistent with a systemic abnormality in fluid and solute regulation in some patients with Meniere's disease, leading to abnormal expansion of the extracellular fluid volume, and possibly contributing to endolympathic hydrops.