Injections of radiocontrast agents are a frequent cause of acute decreases in renal function, occurring most often in patients with chronic renal insufficiency and diabetes mellitus. We prospectively studied 78 patients with chronic renal insufficiency (mean [+/- SD] serum creatinine concentration, 2.1 +/- 0.6 mg per deciliter [186 +/- 53 mumol per liter]) who underwent cardiac angiography. The patients were randomly assigned to receive 0.45 percent saline alone for 12 hours before and 12 hours after angiography, saline plus mannitol, or saline plus furosemide. The mannitol and furosemide were given just before angiography. Serum creatinine was measured before and for 48 hours after angiography, and urine was collected for 24 hours after angiography. An acute radiocontrast-induced decrease in renal function was defined as an increase in the base-line serum creatinine concentration of at least 0.5 mg per deciliter (44 mumol per liter) within 48 hours after the injection of radiocontrast agents. Twenty of the 78 patients (26 percent) had an increase in the serum creatinine concentration of at least 0.5 mg per deciliter after angiography. Among the 28 patients in the saline group, 3 (11 percent) had such an increase in serum creatinine, as compared with 7 of 25 in the mannitol group (28 percent) and 10 of 25 in the furosemide group (40 percent) (P = 0.05). The mean increase in serum creatinine 48 hours after angiography was significantly greater in the furosemide group (P = 0.01) than in the saline group. In patients with chronic renal insufficiency who are undergoing cardiac angiography, hydration with 0.45 percent saline provides better protection against acute decreases in renal function induced by radiocontrast agents than does hydration with 0.45 percent saline plus mannitol or furosemide.