The aim of this prospective observational study was to assess the yield of routine fine-cut computed tomographic (CT) scans in patients with suspected basal skull injuries. Over an 8 month period in 1994, 500 consecutive head-injured patients were examined for clinical signs of basal skull fracture and underwent fine-cut (5 mm) CT scans through the skull base in addition to standard (10 mm) cuts through the vault. Clinical signs were present in 144 patients (144/500, 28.8 per cent) of which 75 (75/144, 52 per cent) had corresponding CT evidence of fracture. A further 22 patients (22/500, 4.4 per cent) had no clinical signs but fractures were seen on CT. The presence of cranial nerve injury, cerebrospinal fluid leak, epistaxis, periorbital bruising, and two or more signs, were more likely to be associated with positive CT scans (P < 0.001, chi 2 tests). The incidence of associated mass lesions was 50.5 per cent, of which 55.1 per cent required craniotomy. This was significantly higher than in patients without evidence of skull base fracture (16.6 per cent) (P < 0.001, chi 2 27.165). Six patients, two of whom had meningitis, required surgical repair of the dural defects seen on CT. The diagnostic yield of routine fine-cut CT scans in this sub-type of head injury is 52 per cent, and is of value in detecting associated mass lesions or significant dural defects which may require surgical intervention.