Two dose levels of ticrynafen, a new uricosuric diuretic, and of hydrochlorothiazide were randomly assigned, double-blind to 240 men with initial diastolic blood pressures in the range of 95 to 114 mm Hg. A dose of 500 mg of ticrynafen once daily exerted an antihypertensive effect comparable to that of 50 or 100 mg of hydrochlorothiazide. Whereas serum uric acid levels rose in patients treated with hydrochlorothiazide, they fell markedly in those receiving ticrynafen. Otherwise, both diuretics produced similar chemical changes in serum. Patients tolerated ticrynafen as well as they did hydrochlorothiazide over a period of six months of observation, and there was no evidence of serious toxicity or loss of therapeutic effect with ticrynafen. This antihypertensive agent, in appropriate doses, appears to be as effective and well tolerated as hydrochlorothiazide, and in addition ticrynafen prevents hyperuricemia.