The effect of large doses of dexamethasone (80 mg) on the cerebrospinal fluid pressure (CSFP) was investigated with continuous lumbar CSFP measurement over 24 h in 10 patients with supratentorial cerebral tumors and clinical signs of elevated intracranial pressure. There was a further slight rise in CSFP with an unchanged pressure pattern in the course of observation of the untreated control group. A marked lowering of CSFP was observed within 6 h after initial intravenous administration of dexamethasone to seven of the treated patients. In four of the treated patients an unequivocal and in five only a moderate CSFP lowering was measured at the end of the registration. One patient had a slight rise in pressure. The plateau waves of all the patients treated decreased in height, frequency and duration within the first hours, while fluctuations in pressure level and CSFP amplitude were reduced.