Twelve infants with hydrocephalus were treated with acetazolamide. In those treated intravenously middle cerebral artery blood flow velocity increased by a median of 86% (range 54-150%). Maximum increase was reached within 2-20 min. The effect lasted as long as 3.5 hours. Intracranial pressure rose by a median of 4 mmHg (range 0-10 mmHg). There was no significant alteration in blood pressure or heart rate. Respiratory rate increased by approximately 10 breaths/min and the tcpCO2 rose by a median 0.2 kPa in infants with normal lungs. In infants treated orally, blood velocity rose by 35-40% at 60-80 min with no increase in intracranial pressure. In four infants with lung disease pCO2 rose by a median of 2.0 kPa (range 0.6 to 3.4 kPa). Acetazolamide was well tolerated in infants with normal lungs but should be used with caution in the presence of lung disease.