Ten adult dry skulls were radiographed in the lateral projection to evaluate the appearance of the opisthion, the posterior margin of the foramen magnum. The opisthion was seen on all 10 skulls and its position was confirmed by outlining it with barium paste. In cross section, it is a teardrop-shaped structure continuous with the internal occipital crest. A beak-shaped shadow anterior to the opisthion was also examined with the barium technique. It is an overlap shdow formed by the interior margin of the foramen magnum and the inferior border of the occipital bone on the same side. In a review of 50 random radiographs of the cervical spine, the opisthion was identified on 42 (84%) and the beak-shaped overlap shadow was seen on 28 (56%). The spinolaminal line, which delineates the posterior margin of the cervical spinal canal, was an unreliable indicator of the position of the opisthion in 17 (40%) of the 42 cases in which the opisthion could be identified.