Pneumoencephalograms of six patients with progressive supranuclear palsy were analyzed with special attention to the midbrain and were compared to studies of patients with Parkinson's disease and those with various cerebellar degenerative diseases. Severe atrophy of the midbrain tegmentum and atrophy of the superior colliculi relative to the inferior colliculi was found in the progressive supranuclear palsy patients and not to the same degree in any of the other groups. These previously unreported findings should help differentiate progressive supranuclear palsy from other degenerative neurologic diseases. Normal standards for midbrain size and shape are also given.