Background Remodeling of the left ventricle with the development of a spherical cavity occurs in dilated cardiomyopathy and is associated with a poor long-term prognosis. The early effects of myocarditis on left ventricular geometry have not been previously described or correlated with clinical outcome. Methods The baseline echocardiograms of 35 patients with biopsy-confirmed myocarditis were compared with 20 normal controls. Left ventricular end-diastolic volume, long axis length, and mid-cavity diameter were measured. The degree of sphericity was expressed as the ratio of the mid-cavity diameter to the long axis length. Left ventricular ejection fraction was assessed by radionuclide angiography. Results In patients with myocarditis, mean left ventricular volume of 81 ± 29 mL/m² was significantly greater than 50 ± 8 mL/m² in controls (P = .001). Chamber dilatation occurred primarily along the mid-cavity diameter, which measured 5.3 ± 0.8 cm in patients with myocarditis versus 4.2 ± 0.4 cm in controls (P = .001). The degree of left ventricular sphericity in patients with myocarditis, 0.64 ± 0.08, was significantly greater than that of controls, 0.54 ± 0.04 (P = .001). When patients were stratified according to left ventricular volume, patients with increased left ventricular volume (>75 mL/m²) were associated with a more spherical chamber and lower left ventricular ejection fraction than patients with a more normal left ventricular volume (≤75 mL/m²). Conclusions Active myocarditis is associated with early left ventricular remodeling and the development of a spherical chamber. These changes correlate with ventricular dilatation and reduced left ventricular ejection fraction. (Am Heart J 1999;138:303-8.)