Several mechanisms are involved in the development of secondary ischemic brain damage, including microthrombi formation, which is thought to play a prominent role. Ninety-four autopsy cases were macro- and microscopically examined by specific staining for fibrin, 74 of which showed cortical contusion after a craniocerebral trauma, Twenty cases with no neurological pathology were used as controls, Traumatic cases comprised 52 males and 22 females, with a mean age of 48 years; most cases died in the first 48 h, The total number of fibrinous microthrombi in a slice of each hemisphere was determined. The mean number of microthrombi found in contused hemisphere was 152 (37-283), with 88 in the contralateral hemisphere (21-139) as compared to 13 (0-27) in control cases, Differences were statistically significant. Globular microthrombi or "shock bodies" (2-60 mu diameter) were present in five cases. Enhanced presence of microthrombi in contused brain areas, higher incidence in young people, an increase in the amount of microthrombi up to the 9th day after injury and involvement of the contralateral hemisphere free of contusion foci were all demonstrated, Microthrombi would therefore seem to be one of the central secondary events after brain trauma to bear in mind when designing treatment strategies.