Acute cerebellar ataxia has been described occasionally with infectious mononucleosis. Two additional cases are reported with serologic identification of Epstein-Barr virus (EBV) infection in blood and cerebrospinal fluid. As with previously described cases, the outcome was benign, and examination and laboratory studies did not indicate diffuse neurologic involvement. Visual and brainstem auditory-evoked responses were normal. Electroencephalograms (EEG) demonstrated 14 and 6 per second positive spikes in both patients. This pattern is considered a normal variant and has been recorded from depth electrodes and reported with deep midline lesions. These cases support the prognosis of benign cerebellar involvement in infectious mononucleosis and suggest that evidence of EBV infection be sought in patients with acute ataxia. The significance of 14/sec and 6/sec positive EEG spikes is uncertain.