B-waves are rhythmic oscillations of intracranial pressure (ICP) with a wavelength of 0.5-2 min. These oscillations are accompanied by corresponding fluctuations of arterial blood pressure (BP). There is controversy in the literature whether the ICP oscillations precede or follow the BP oscillations. This relationship, however, is important for the understanding of the origin and the interactions of ICP B-waves with other parameters. ICP and BP were monitored continuously for one night in nine patients with suspected symptomatic normal pressure hydrocephalus (NPH). Forty-five B-waves were identified and the time delays between the peaks and the onsets of B-waves and their BP-equivalents were measured. The mean (+/- standard deviation) of the time delays between the peaks of ICP B-waves and corresponding BP-oscillations was 4.2 +/- 2.4 sec and between the onsets of the oscillations 10.3 +/- 6.6 sec. In all but two B-waves, the ICP peak preceded the BP peak, in all the 45 B-waves the onset of the ICP increase preceded the BP increase (p < 0.004). Our study demonstrates that ICP B-waves precede concomitant changes in BP by several seconds.