Thirty-two hospital patients in advanced states of fluid retention from heart failure, cirrhosis, and nephrosis were given a new sulfonamide compound, chlorothiazide, to induce diuresis. Chlorothiazide, given orally in a dosage of 2.0 Gm. per day, was found to be at least as potent as an organic mercurial agent in promoting diuresis. The compound was effective in all types of edema studied, a finding which perhaps supports the hypothesis that similar mechanisms operate in the abnormal renal retention of sodium in heart failure, hepatic cirrhosis, and nephrosis. Chlorothiazide promises to be useful whenever reduction in stores of body sodium is desirable, and therefore may also be of value in hypertension and in steroid edema. In addition, the compound may contribute to a further understanding of renal tubular mechanisms involved in edema formation, since the study suggests a site of action different from other known diuretics.