A prospective study of 150 CT scans showed that the right ethmoidal roof was lower than the left in 8.6 per cent of cases. The reverse situation, i.e. the right higher than the left, was observed in only 1.2 per cent of cases. This finding could account for the higher reported incidence of endocranial complications associated with right ethmoidectomy. Coronal CT scans allowing comparison of the right and left ethmoidal roofs should always be made before undertaking intranasal ethmoidectomy.