Background: B waves, slow and rhythmic oscillations in intracranial pressure (ICP), are claimed to be one of the best predictors of outcome after surgery for normal pressure hydrocephalus (NPH). Object: To determine the relation between the percentage of B waves and outcome in patients with hydrocephalus, and also t he diurnal variation of B waves. Methods: ICP and patient behaviour were recorde d overnight (17 to 26 hours) in 29 patients with non- communicating hydrocephal us and 26 with NPH. The B wave activity, measured with an amplitude threshold of 0.5, 0.75, 1.0, 1.5, 2.0, 3.0, and 5.0 mm Hg, was estimated as the percentage o f total monitoring time (% B waves) using a computer algorithm, and correlated with postoperative outcome, defined as changes in 12 standardised symptoms and signs. Results: There was no linear correlation between improvement after surger y in the 55 patients and total % B waves, but a correlation was found between improvement and % B waves during sleep (r=0.39, p=0.04). The percentage of B w aves was the same during sleep and wakefulness, and patients with NPH had the sa me proportion of B waves as the non- communicating patients. Conclusions: B wav es are commonly observed in patients with both communicating and non- communica ting hydrocephalus, but are only weakly related to the degree of postsurgical im provement. Background: B waves, slow and rhythmic oscillations in intracranial pressure (ICP), are claimed to be one of the best predictors of outcome after surgery for normal pressure hydrocephalus (NPH). Object: To determine the relation between the percentage of B waves and outcome in patients with hydrocephalus, and also he diurnal variation of B waves. Methods: ICP and patient behavior were recorded for d overnight (17 to 26 hours) in 29 patients with non-communicating hydrocephal us and 26 with NPH. The B wave activity , measured with an amplitude threshold of 0.5, 0.75, 1.0, 1.5, 2.0, 3.0, and 5.0 mm Hg, was estimated as the percentage of total monitoring time (% B waves) using a computer algorithm, and correlated with postoperative outcome, defined as changes in 12 standardized symptoms and signs. Results: There was no linear correlation between improvement after surger y in the 55 patients and total% B waves, but a correlation was found between improvement and% B waves during sleep (r = 0 .39, p = 0.04). The percentage of B w aves was the same during sleep and wakefulness, and patients with NPH had the sa me proportion of B waves as the non-communicating patients. Conclusions: B wav es are commonly observed in patients with both communicating and non- communica ting hydrocephalus, but are only weakly related to the degree of postsurgical im provement.