We present the case of a boy with pulmonary atresia (PA), intact ventricular septum (IVS), and a coronary fistula between the right ventricle (RV) and a single left coronary artery (SLCA). In the newborn period, the child was operated on using a right Blalock–Tauss ing shunt (RBTS). At the age of 6 months, he was admitted to the department of Paediatric Cardiology due to hypoxaemia and cardiac catheterisation was performed before qualification for cardiac surgery. During haemodynamic evaluation of the patient, we conducted the balloon occlusion test of the coronary fistula and demonstrated that coronary circulation depends on the wide fistula and high pres sure in the RV. We decided not to perform embolisation of the fistula and qualified the child for bidirectional Glenn palliation without RV decompression. The balloon occlusion test in patients with coronary fistulas plays a critical role and its result can be crucial for further management of the patient.