The brain scans and clinical records of 25 patients with posterior fossa infarction were reviewed. In four patients with a clinical diagnosis of cerebellar hemisphere infarction, the brain scans showed focally increased uptake of 99Tc(M) pertechnetate in the affected hemisphere. In 21 patients with a clinical diagnosis of brainstem infarction, the brain scans were normal. In one patient who underwent serial scanning, the evolution of the scan abnormality was similar in time course to that described for cerebral hemisphere infarction. There was no apparent correlation between scan abnormality and prognosis for neurologic recovery.