Background Elevated levels of fibroblast growth factor 23 (FGF23) are associated with increased risk of adverse outcomes in patients withCKD. Reducing dietary phosphate intake or absorptionmay decrease FGF23 levels, but data on the combined effects of dietary phosphate restriction and phosphate binders in CKD are limited. Design, setting, participants, & measurements In this 2×2 factorial, single-blinded, placebo-controlled, 3-month study, conducted between July 2009 and March 2012, 39 patients with CKD stages 3 or 4 and normal serum phosphate levels were randomly assigned to one of four groups: ad libitum diet plus lanthanum carbonate (LC) placebo (n=10), 900-mg phosphate diet plus LC placebo (n=10), ad libitum diet plus LC (n=11), or 900-mg phosphate diet plus LC (n=8). The dose of LC was 1000 mg three times daily with meals. Dietary restriction was accomplished with outpatient counseling. The primary end point was change in FGF23 levels from baseline. Results Compared with ad libitum diet, the 900-mg phosphate diet did not significantly reduce FGF23 levels (diet × time interaction, P=0.05). Compared with placebo, LC alone also did not significantly reduce FGF23 levels (LC × time interaction, P=0.21). However, the dual intervention significantly decreased FGF23 levels throughout the study period (diet × LC × time interaction, P=0.02), resulting in a 35% (95% confidence interval, 8%-62%) reduction by study end. Conclusion The combination of LC plus counseling for a phosphate-restricted diet decreased FGF23 levels in patients with CKD stages 324 and normal serum phosphate levels. © 2013 by the American Society of Nephrology.