The carbonaceous oral adsorbent AST-120 slows the deterioration of kidney function in patients with advanced chronic kidney disease (CKD). However, information about AST-120 in patients with less severe stages of CKD is lacking. Randomized controlled trial. 75 medical facilities, 460 patients with CKD with serum creatinine (sCr) concentrations less than 5.0 mg/dL (not undergoing dialysis). Random assignment to either a low-protein diet and antihypertensive medication in the control group or that treatment combined with AST-120 (6 g/d). Composite primary end point: doubling of sCr level, increase in sCr level to 6.0 mg/dL or more, need for dialysis or transplantation, or death. adverse events and changes in estimated creatinine clearance (CCr) rate, proteinuria (protein in milligrams per day), and quality of life. Mean sCr level was 2.66 mg/dL and estimated CCr was 22.4 mL/min in both groups. During 56 weeks, numbers of primary end-point events (43 for control versus 42 for AST-120) and event-free survival (P = 0.9) did not differ between groups. Gastrointestinal adverse events were less common in the control group than the AST-120 group (2 versus 32 events). Estimated CCr decreased more in the control group than in the AST-120 group (-15% per year versus -12% per year, relative to the baseline value; [corrected] P = 0.001). Median proteinuria changed from protein of 1,162 to 1,167 mg/d in the control group versus 1,102 to 906 mg/d in the AST-120 group (P = 0.2). Infrequent primary end-point events. AST-120 did not substantially slow the progression of kidney disease in patients with moderate to severe CKD during 1 year.