Isolated enlargement of the right chambers of the heart is uncommon in adults. It usually occurs secondary to subacute bacterial endocarditis involving the tricuspid or pulmonic valves (or both) and often is seen in drug addicts. The authors present an uncommon case involving a patient with AIDS and tricuspid endocarditis associated with isolated severe right atrial dilatation. Planar adenosine stress TI-201 myocardial perfusion imaging showed a large photopenic defect in the lower right chest in the region of the right atrium. First-pass and gated blood pool studies showed right atrial enlargement. TI-201 myocardial imaging confirmed the severe dilatation of the right atrium, which appeared as a photopenic area. The study also revealed a prolonged pulmonary transit time of 12 seconds.