More frequent hemodialysis sessions and longer session lengths may offer improved phosphorus control. We analyzed data from the Frequent Hemodialysis Network Daily and Nocturnal Trials to examine the effects of treatment assignment on predialysis serum phosphorus and on prescribed dose of phosphorus binder, expressed relative to calcium carbonate on a weight basis. In the Daily Trial, with prescribed session lengths of 1.5-2.75 hours six times per week, assignment to frequent hemodialysis associated with both a 0.46 mg/dl decrease (95% confidence interval [95% CI], 0.13-0.78 mg/dl) in mean serum phosphorus and a 1.35 g/d reduction (95% CI, 0.20-2.50 g/d) in equivalent phosphorus binder dose at month 12 compared with assignment to conventional hemodialysis. In the Nocturnal Trial, with prescribed session lengths of 6-8 hours six times per week, assignment to frequent hemodialysis associated with a 1.24 mg/dl decrease (95% CI, 0.68-1.79 mg/dl) in mean serum phosphorus compared with assignment to conventional hemodialysis. Among patients assigned to the group receiving six sessions per week, 73% did not require phosphorus binders at month 12 compared with only 8% of patients assigned to sessions three times per week (P<0.001). At month 12, 42% of patients on nocturnal hemodialysis required the addition of phosphorus into the dialysate to prevent hypophosphatemia. Frequent hemodialysis did not have major effects on calcium or parathyroid hormone concentrations in either trial. In conclusion, frequent hemodialysis facilitates control of hyperphosphatemia and extended session lengths could allow more liberal diets and freedom from phosphorus binders.