To study whether the renin-angiotensin system is related to hyperuricemia in hypertension, the serum concentration of uric acid was determined in 96 patients with various types of hypertension and various degrees of plasma renin activity (PRA). In malignant hypertension, both PRA and the serum uric acid level were higher than in essential hypertension; but in primary aldosteronism or desoxycorticosterone-excess hypertension, they were lower than in the essential type. In renovascular hypertension, PRA was higher than in essential hypertension, but the serum uric acid levels were similar. There were no differences in PRA and serum uric acid concentration between Cushing's syndrome and essential hypertension. The serum uric acid level in high-renin essential hypertension was higher than in either the normal-renin or the low-renin type. There was a significant correlation between serum uric acid concentration and PRA in the basal state, and between the change in PRA and the change in serum uric acid induced by administration of furosemide. Apparently the close correlation between the renin-angiotensin system and the concentration of serum uric acid is related to changes in extracellular fluid volume, although an intrarenal effect of angiotensin II cannot be excluded.