The general classification of head injury proposed by Marshall et al., based on admission CT scan findings, might mask a group of patients who have Diffuse Brain Injury (DI) in addition to intracranial haematomas. The aim of this study was to assess possible differences in outcome with respect to the level of intracranial pressure (ICP) and cerebral perfusion pressure (CPP) between a group of patients with DI: III-IV (Marshall's classification) after the evacuation of an intracranial haematoma (group A) and another group with DI: III-IV in the absence of a mass lesion (group B). We prospectively studied 129 patients with isolated and closed severe head injury (GCS < 9). In group A (n = 61), the median percentage of hours with ICP > 20 mmHg and CPP < 70 mmHg was 42.8 and 18, respectively and 17 (28%) survived with GOS 4-5. In group B (n = 68), median values of 20 and 5.5 hours were obtained for ICP > 20 and CPP < 70 respectively, whilst 39 (57.3%) survived with favourable outcomes. When we analysed the effects of the DI: III-IV in both groups of patients, we found that the differences in percentage of time with ICP > 20 and CPP < 70 were statistically significant (p < 0.01) and patients in group A had a higher morbidity and mortality (p < 0.05). This study has demonstrated that the levels of ICP, morbidity and mortality in patients with DI: III-IV and an evacuated mass lesion were higher than in patients with DI: III-IV without a mass lesion.