Pre- and post-operative assessments are presented in 17 adult patients who have been treated with craniovertebral decompression for hindbrain herniation, 11 of whom had syringomyelia. Objective improvement in the size of the syrinx was seen in all 11 cases; contrary to expectation the hindbrain more frequently moved downwards than upwards after decompression of the tonsils and creation of an artificial cisterna magna. A method is reported for the quantitative assessment of hindbrain migration using magnetic resonance imaging (MRI). The degree of 'slump', further downward displacement of the hindbrain, was compared with the clinical outcome and the MRI appearances. Slump was more common than expected, although the severity was not usually great enough to produce symptoms. Slump was less marked where the artificial cisterna magna was generous (p less than 0.02). This quantitative method may be useful in assessing patients with unexplained post-operative symptoms and in comparing different surgical techniques.