Understanding of the dynamic changes in cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO2), and intracranial pressure (ICP) is of utmost importance in the clinical care of patients with severe head injury (HI). Thus, four principles of treatment in common use in the management of patients with HI (i.e. prolonged artifical hyperventilation, barbiturate sedation, hypothermia and mannitol treatment) are based on the principles for regulation of CBF, CMRO2, and ICP. During the last decade a multitude of studies concerning the dynamic changes in CBF, CMRO2, and ICP have been published. These studies have been supplemented with studies of cerebral autoregulation (CA) and studies of the chemical regulation of CBF, especially CO2 reactivity. Recently the topic has been reviewed (Enevoldsen 1980, Jennett and Teasdale 1981, Enevoldsen 1986, Sundbärg 1988); however, the therapeutical implications of clinical CBF studies have only rarely been discussed. In this review experimental and clinical studies of cerebral circulation and metabolism in severe head injury have been supplemented with studies of intracranial pressure (ICP), biochemical studies of brain tissue and cerebrospinal fluid, and the theoretical implications for therapy are discussed. Electroencephalographic investigations, studies with transcranial doppler technique, studies of magnetic resonance and CT scanning have only been considered if they elucidated the dynamic changes in CBF, ICP or metabolism.