Intra-aortic balloon counterpulsation became ineffective in a 63 inch (160 cm) 14 year old girl while console operation proceeded almost normally. The problem was simulated in a hydraulic model of the aorta and could be reproduced when the proximal end of the balloon engaged in the introducing sheath. Measurements of the balloon catheter and sheath against the patient indicated that such engagement had occurred during counterpulsation. The findings provide an explanation for apparent catheter 'kinkage' in other patients where there was difficulty running the Kontron console in automatic mode. Problems are preventable by total or partial withdrawal of the inserting sheath.