Tension pneumocephalus is an intracranial collection of gas under pressure which is a complication of posterior fossa surgery performed in the sitting position. Characteristically, this syndrome is suspected because of delayed return to consciousness and neurologic deterioration in the postoperative period. Diagnosis can be confirmed by computerized tomographic (CT) scan. Hemodynamic instability progressing to cardiac arrest due to entrapped gas has not been reported. The authors describe a case involving a 5-year-old child who sustained a sudden cardiac arrest apparently caused by tension pneumocephalus.