Eight Japanese patients with Joseph disease were studied using computed tomography (CT). Morphometric analysis using the two-dimensional (area) measurement by CT was performed in the infratentorial region. The brain-stem index, an index of brain-stem atrophy or pontine atrophy, revealed a significant decrease (P less than 0.01), with a mean of 66.7% when compared with 16 control subjects. The patients showed a significant increase (P less than 0.01), with a mean of three times that of the controls in the fourth ventricular index, an index of fourth ventricular dilatation. There were no differences in the cerebellar index, an index of cerebellar atrophy, between these patients and the controls, although the patients had an increased number of visible cerebellar vermian (2.0, SD 0.7) and hemispheric sulci (2.6 SD 0.6) as compared with the controls (vermian sulci: 0.4, SD 0.7: hemispheric sulci: 0). These data indicate severe pontine atrophy, fourth ventricular dilatation with mild involvement of the cerebellum and correlate well with the common pathological features of Joseph disease. The present morphometric evaluation by CT may be useful in the clinical diagnosis of Joseph disease.