CT scans of the nasopharynx and the base of skull were performed in 54 patients with histologically proven NPC. The CT findings are compared to those obtained by routine clinical examination. In this series, involvement of the parapharyngeal spaces was found by CT scan in 61% (33/54) of patients, which is impossible to assess by clinical means. Intracranial invasion was demonstrated by CT scan with intravenous contrast enhancement in five patients. CT is more sensitive in showing the erosion of skull base than X ray film. CT scans demonstrated 17% (5/30) of bony invasion in X ray negative patients. Clinical examination showed one involved wall in nine patients; CT scan discovered involvement of two or more walls in five (55%) of these nine. Routine clinical examination discovered extra-nasopharyngeal extensions in 34 locations, whereas CT scans discovered 106, a ratio of 1:3.1. The common sites of extra-nasopharyngeal extension detected by CT scan are: parapharyngeal space, intracranial invasion, sphenoidal sinus, orbit, ethmoidal sinus, maxillary antrum, oropharynx and the nasal cavity. In T-staging of NPC, 55% (6/11) of clinical T2 lesions are up-staged by CT. Thirty-six percent (4/11) of clinical T3 lesions were also up-staged. However, 94% (17/18) of clinical T4 lesions remained unchanged. Only 6% (1/18) of clinical T4 lesions were down-staged.