The pathogenesis of posttraumatic subdural hygroma still remains largely unknown. One of the suggested pathological mechanisms is the traumatic development of an arachnoid tear and the subsequent efflux of CSF into the subdural space. We performed a multifactorial analysis of the hygroma fluid obtained at operation in comparison to the simultaneously taken plasma and lumbar CSF. The results of the protein analysis support the CSF origin of the subdural hygroma fluid.