Background: AST-120 is an orally administered adsorbent used in Japan for prolonging time to initiation of hemodialysis and improving uremic symptoms in patients with chronic kidney disease (CKD). As AST-120 is suspected to reduce the progression of CKD by adsorbing renal toxins in the gastrointestinal tract, the objective of the current study was to determine whether binding of AST-120 to creatinine in the intestines could acutely alter creatinine balance, thereby limiting the utility of serum creatinine (sCr) as a measure of progression of renal function. Such information may be critical for the design of future studies to assess the efficacy of AST-120 in CKD patients. Methods: Patients with CKD (n = 20) received oral doses of AST-120(3 g t.i.d.) and placebo in a two-way crossover study. Blood and urine were collected for determination of sCr, 24-hour urinary creatinine (UcrV), creatinine clearance (Ccr), and urea nitrogen clearance (URCL). Differences between treatments were assessed using an ANCOVA model. Results: Following AST-120 and placebo treatments, mean sCr (1.73 and 1.79 mg/dl, respectively) and UcrV (1,264.73 and 1,286.05 mg) values were not significantly different. No significant differences were observed for Ccr and URCL. Conclusion: These results indicate that AST-120 has no acute impact on creatinine balance in patients with CKD. Consequently, sCr and other markers of renal function are acceptable measures for assessing changes in renal function following AST-120 treatment.