We report a case of tension pneumoventricle in a patient who underwent excision of a midbrain tectal plate tumour via a supracerebellar infratentorial approach while in the sitting position. Tension pneumoventricle was diagnosed when the patient's clinical condition deteriorated rapidly early in the postoperative period. An urgent computed tomography scan of the head revealed the presence of air in the frontal and temporal horns of the lateral ventricles and blood in the third and fourth ventricles. An emergency external ventricular drain insertion was performed, resulting in prompt recovery. We believe the contributing factors resulting in such rapid development of tension pneumoventricle were the patient's sitting position, pre-existing hydrocephalus, breach of the third ventricle during surgery and the presence of blood in the third and fourth ventricles.