Exercise 201Tl SPECT imaging has become routinely accepted and utilized as a major screening test for atherosclerotic coronary artery disease. In appropriate clinical situations, an abnormal 201Tl study usually will require a subsequent coronary angiogram to confirm the presence of an abnormality and to define its pathologic anatomy. Although most reversible thallium defects will prove to be secondary to significant coronary artery atherosclerosis, congenital coronary or cardiac anomalies can occasionally be responsible, and it is useful to be aware of these, particularly in the evaluation of relatively young symptomatic patients. We report, for the first time, a coronary arteriovenous fistula as the cause for an exercise-induced reversible 201Tl perfusion abnormality.