Many anatomical factors in difficult intubation at direct laryngoscopy have been evaluated. Lateral radiographs were taken of nineteen patients in whom tracheal intubation proved particularly difficult, and fourteen patients whose intubation was reasonably straightforward. Stepwise discriminant analysis was used to select the best measurements for distinguishing between the difficult and straightforward groups. The variables which together are most reliable in predicting likely difficulty in intubation are reduced atlanto-occipital extension, reduced mandibular space, and lastly, increased antero-posterior thickness of the tongue. A formula and graph have been derived to relate these variables with likelihood of difficulty, and a method has been described of applying this information at the bedside, without using X-ray examination, to estimate the likelihood of difficulty in intubating a new patient. Eighteen months' experience of the application of this clinical evaluation have so far found it reliable.