OBJECTIVE To describe the rare problem of severe bronchospasm occurring during cardiopulmonary bypass in a six-year-old-child. METHODS Severe bronchospasm became apparent on attempting to resume controlled ventilation prior to weaning from cardiopulmonary bypass. The patient had a previous history of asthma but was asymptomatic preoperatively. Aggressive therapy with multiple bronchodilating agents was necessary before cardiopulmonary bypass could be discontinued. The bronchospasm resolved over the first 24 hr after surgery. CONCLUSIONS Severe bronchospasm during cardiopulmonary bypass is rare. It should only be diagnosed after ruling out other reasons for failure to ventilate. Treatment with intravenous bronchodilators is required. The cause is unknown.