We sought to clarify whether a single-bolus intravenous administration of sodium bicarbonate in addition to hydration with sodium chloride prevents contrast-induced nephropathy (CIN). One hundred forty-four patients with mild renal insufficiency (serum creatinine >1.1 to <2.0 mg/dl) undergoing an elective coronary procedure were randomly assigned to the following 2 groups: standard hydration with sodium chloride plus single-bolus intravenous administration of sodium bicarbonate (20 mEq) immediately before contrast exposure (group A, n = 72) and standard hydration alone (group B, n = 72). The primary end point was development of CIN, defined as an increase >25% or >0.5 mg/dl in serum creatinine within 3 days after the procedure. Incidence of the primary end point was lower in group A than in group B (1.4% vs 12.5%, p = 0.017). Incidence of adverse clinical events (acute pulmonary edema, acute renal failure requiring dialysis, and death within 7 days of procedure) did not differ between the 2 groups (0% vs 1.4%). In conclusion, single-bolus intravenous administration of sodium bicarbonate in addition to standard hydration can more effectively prevent CIN than standard hydration alone in patients with mild renal insufficiency undergoing an elective coronary procedure.