Nuclear magnetic resonance (NMR) imaging can be of value in distinguishing vascular and nonvascular lesions in the lung and mediastinum, in that rapidly flowing blood results in little or no NMR signal. We wish to report a patient having a pulmonary arteriovenous fistula evaluated using physiological studies, dynamic computed tomography, and spin-echo NMR. On the NMR images, the fistula was invisible because of rapid flow through its lumen.