Cytology, lactate dehydrogenase, and creatine kinase in cerebrospinal fluid were investigated 6-72h after mild head injuries in 59 patients aged 16-49 years. Significant differences were not found in creatine kinase. Lactate dehydrogenase, however, was significantly more elevated in mild cerebral contusions compared with concussions 24h after head injury. Monocytoid reaction in cerebrospinal fluid was significantly more distinct in mild contusions compared with concussions 24h after impact. Large phagocytes were rarely found in concussions and were not traceable 48h after impact. Large phagocytes, however, were found in 50-55% of the mild contusions 48-72h after head injury. Siderophages were found only in contusions (16.5-35%), 48-72h after impact). The optimal time for lumbar punction is 24h after head injury.