Regional cerebral blood flow (rCBF) was investigated in 12 patients with brain tumors, using a 254-channel dynamic gamma camera. In nine of the 12 cases, hyperemic regions with loss of autoregulation were seen in sites remote from the tumor (the area around the tumor was in most cases also hyperemic). These remote rCBF abnormalities were found in the lower posterior part of the hemisphere in six cases, and in the frontal region in three. The location of the remote rCBF abnormality seemed to depend on the site of the tumor: cases with frontal and posterior fossa mass lesions had hyperemia in the lower part of the temporooccipital regions, cases with centroparietal mass lesions had hyperemia mostly in the frontal region. This may mean that the remote rCBF abnormality is due to local tissue compression against unyielding anatomical structures, namely, the tentorium and the falx. It is suggested that these abnormalities may constitute evidence of an early stage of a dangerous clinical condition: a state of preherniation.