We retrospectively assessed the incidence and time course of enlargement in post-traumatic intracerebral haematoma (PTICH). Computed tomography (CT) scans from 165 patients who underwent a scan within 72 h and a repeat scan within 120 h of the onset of trauma were examined. A semi-automated method using region deformation-based segmentation was used to calculate the haematoma volume. The presence of haematoma enlargement was also determined based on a consensus by five observers. Seventy cases (42%) showed enlargement of the haematoma. The frequency of haematoma enlargement decreased as the interval between the onset of trauma and the initial scan increased. The discriminant value of the ratio of the haematoma volume in the second scan to that in the initial scan was ascertained, and the cut-off value for haematoma enlargement was determined to be 1.45. The radiographic criterion for enlargement in PTICH on CT scan was, therefore, defined as a > or = 1.45 times increase in haematoma volume.