A 15-year-old girl presented with persistent fevers, night sweats, leukocytosis, an elevated erythrocyte sedimentation rate, and a 13-pound weight loss over 2 months. Duplex Doppler scans, computed tomographic scan, and magnetic resonance imaging studies were suggestive of Takayasu's arteritis. Left ventricular dysfunction occurred during the episode of active disease, and an endomyocardial biopsy demonstrated increased HLA-DR (human leukocyte antigen-DR) on the endothelium and evidence of immune complex deposition in the walls of small vessels. One year later, after treatment with corticosteroids and resolution of clinical symptoms, repeat endomyocardial biopsy revealed focal interstitial fibrosis and persistent immune complex deposition. These results indicate that the inflammatory, vasculitic process affecting the large vessels in Takayasu's arteritis may also involve the endomyocardium and its small vessels resulting in ventricular dysfunction.