Twenty patients with severe head injury were analysed as to whether they developed a humoral immunodeficiency syndrome. In spite of the fact that 75% developed pyogenic complications all patients showed a polyclonal humoral activation of their immune system with an increase in IgG, IgA, IgM and the complement factors. Four patients died from cerebral complications partly associated with infections. Because our patients did not develop a humoral immune defect and because there is no proof so far of a cellular defect, we think that the cause of respiratory infections is a pulmonary disturbance primarily caused by hypothalamic factors, whose release is stimulated by traumatically induced cerebral lesions.