The authors introduce a two-dimensional scale for rating closed-head injury, the Head Injury Severity Scale (HISS). This system is based on a five-interval severity classification (minimal through critical), determined primarily by the initial post-resuscitation Glasgow Coma Scale score. The second dimension is predicated on the presence or absence of complications, appropriate for each severity interval. The outcomes of almost 25,000 patients with head injury encountered at our institution over a 7-year period were evaluated. We discovered that adding a complication dimension to each severity category resulted in significant outcome differences and effectively divided patients into groups with very different risks, prognosis and treatment requirements. The HISS is proposed as a framework on which further research can be done to guide care to predict outcome and to perform audits on head-injured patients.