The properties of a new diuretic, bumetanide, were investigated in ten normal volunteers, and sixty-eight patients with fluid retention. It had a rapid action virtually complete in 4 hr with an effect on water and electrolyte excretion similar to that of frusemide. It was highly potent, 1 mg producing an effect comparable to 48 mg frusemide in normal subjects, and about 40 mg in oedematous subjects in controlled short-term experiments. In long-term studies in forty-eight patients extending over periods up to 51 weeks it was found to be effective in controlling oedema. There was a low incidence of electrolyte complications and no other biochemical abnormalities or haematological complications were attributed to the drug. Bumetanide was well tolerated by patients. A drug rash was observed in two cases.