Analysis of monitoring records of 150 patients given over 1000 infusions of 20% mannitol delineared three variables affecting the response of intracranial pressure (ICP) to mannitol: the original ICP; the current dose; and the dose given over the preceding three hours. The level of ICP influenced the response to mannitol as much as the amount of mannitol; giving more mannitol than was required to bring ICP below 25 mm Hg led to the need for larger following doses. One hundred-milliliter-bolus doses were often as effective as larger doses. The results suggest that doses of mannitol given to control increased ICP should be kept as small as possible.