The study of a series of brains from patients who had a severe head injury and died within 72 h without a lucid interval showed that there was a step-wise progression in the development of retraction balls. At 2 h after injury sinusoidal enlargement of the axons was evident. This progressed over 16 h when the lesions appeared as retraction balls which were fully developed at 72 h. There was a similar increase of staining with an immunoperoxidase method for glial fibrillary acid protein (GFAP) initially around blood vessels spreading diffusely into the white matter. The number of reactive astrocytes also increased. In a control case where the corpus callosum was torn at post-mortem there were sinusoidally distended and torn axons in the absence of GFAP staining. It is proposed that there are three components to a head injury. First, mechanical injury as seen in the control case; second, the development of retraction balls which are an active process probably representing damaged axons which cannot undergo repair where the sinusoidal swellings develop into retraction balls and third, an astrocytic reaction. The sinusoidal change, when present on its own, may not be separable from post-mortem trauma. However, when it is associated with an astrocytic response it should be correlated with coma in the same way as retraction balls.