The extensive use of mannitol during maintenance hemodialysis prompted a study of mannitol kinetics. In 17 patients receiving empirical mannitol therapy of mannitol kinetics. In 17 patients receiving empirical mannitol therapy, residual levels range from 19 to 100 mg% In vitro mannitol clearance exceeds 125 ml/min for the CDAK 5 and PF 1.6 dialyzers, while in vivo clearance ranges from 98 to 140 ml/min in the Triex 1, PF 1.6 and CDAK 5. Despite an apparently adequate clearance rate, mannitol administered during dialysis is incompletely removed. Repeated use of mannitol during dialysis leads to mannitol accumulation. Clinical significance of the residual mannitol levels needs further evaluation.