Icodextrin use during the long dwell of a peritoneal dialysis (PD) regimen is commonly used to increase ultrafiltration. Its use may cause a mild and clinically insignificant degree of hyponatremia. We describe a patient who was admitted twice to our medical center on an atypical continuous ambulatory peritoneal dialysis (CAPD) regimen utilizing solely icodextrin with 2 exchanges (12-hour dwells). On both admissions, he had hyperosmolar hyponatremia in the 120-mmol/L range with a large osmolal gap. After icodextrin was stopped and his PD prescription was switched to dextrose solutions, both hyponatremia corrected and the osmolal gap quickly disappeared. The accumulation of osmotically active solute in extracellular fluids results in efflux of water from the cellular compartment and produces both hyponatremia and hypertonicity [ 1 ]. This tonic effect occurs most frequently with hyperglycemia, but other substances can also cause this, including mannitol, sorbitol, glycine, and maltose [ 1 , 2 ]. In this report, we present a patient with end-stage renal disease (ERSD) on an atypical off-label PD regimen utilizing solely icodextrin solutions who developed hyperosmolar hyponatremia in the 120-mmol/L range, with a large osmolal gap. This appeared to be due to absorbed metabolites of icodextrin, mainly maltose.