In a man aged 65 years undergoing diagnostic coronary arteriography, a fistula was found arising from the circumflex coronary artery and entering a branch of the left pulmonary artery (panel A, black arrows), while exercise stress myocardial perfusion scintigraphy (Th²⁰¹ SPECT) showed normal distribution of the radiotracer. Estimation of blood flow using a Doppler guidewire (panel A, white arrows) revealed that the flow through the fistula was mainly during systole (panel B, diastolic to systolic velocity ratio (DSVR) 0.3), while the flow in the circumflex was mainly during diastole (panel C, DSVR 3.2). The latter provides a probable explanation of the Th²⁰¹ SPECT findings, indicating that the fistula did not affect the myocardial perfusion.