Plateau waves, or Lundberg “A” waves, have been recognized since 1960. However, no comprehensive theory to explain why they occur has been presented. Explanations have generally been based upon the occurrence of vasodilatation which led to an increase in cerebral blood volume (CBV) and a subsequent rise in the intracranial pressure. This increase in blood volume has been demonstrated by Risberg et al. (1969) who also suggested that the increase was maximal just prior to termination of the wave. Previous investigators have also recognized the necessity of decreased intracranial compliance before plateau waves will occur (Lundberg 1960, Matsuda et al. 1979).