Indium-111 monoclonal antimyosin antibody imaging is a convenient method to diagnose acute myocarditis. The present study examined the natural evolution of a positive antimyosin scan in 10 patients with acute myocarditis, in relation to left ventricular ejection fraction. All patients underwent myosin-specific monoclonal antibody imaging and radionuclide ventriculography; in all patients endomyocardial biopsy was performed. Repeat assessment of left ventricular function and antimyosin uptake was performed 1, 2 and 6 months later. A heart-to-lung ratio was used to quantify antimyosin uptake. Heart-to-lung ratio in patients with myocarditis was 2.12 +/- 0.31, significantly higher than that observed in eight controls (1.35 +/- 0.08, P < 0.001). During follow-up 2.2 repeat scans per patient were performed. Heart-to-lung ratio normalized (< 1.6) within 1 month in four patients, within 2 months in one patient and within 6 months in two patients; three patients continued to have a positive antimyosin uptake at 6 months, suggesting ongoing myocardial necrosis. At the end of follow-up, ejection fraction improved by > 5% in four patients; normalization or persistence of tracer uptake could not predict an improvement of ejection fraction. It is concluded that antimyosin scintigraphy is useful for diagnosing myocarditis in its subacute or chronic stage during which unknown or immune mechanisms may be in operation. An improvement in left ventricular function cannot be predicted on the basis of persistence or not of tracer uptake.