BACKGROUND: Significant elevations of serum sialic acid level have been documented in various diseases including in a variety of central nervous system disorders. But, in head injury, there is no any study on the serum and brain tissue sialic acid levels. So, we planned an experimental study to evaluate serum and brain tissue sialic acid levels in head injury. METHODS: Marmarou's impact-acceleration model was used in rats to produce diffuse brain injury. Rats were divided into equal three groups. In Group I, 450 g weight was fell from 1 m height to heads of subjects, and from 2 m in Group II. Group III was control group. Sialic acid levels were measured in both sera and brain tissue supernatants after trauma. RESULTS: It was observed that serum sialic acid level was decreased according to the severity and period of trauma increased; and there was no change in brain tissue sialic acid levels. CONCLUSION: Serum sialic acid level might be used as a marker to show the degree of diffuse brain injury.