Pathophysiological and CT aspects of wallerian degeneration of the corticospinal tract in the brain stem were studied in patients with old hemispheric infarcts. There was found to be a clear relationship between (a) the location and size of the infarcts and (b) the presence or absence of wallerian degeneration. When most of the motor cortex is involved, degeneration can be seen at least as far as the mesencephalic level and sometimes down to the pontine level. Isolated capsular infarcts which spare the motor cortex are also associated with degeneration. No degeneration could be seen in patients with small infarcts.