Twenty patients with decreased left ventricular (LV) function and endomyocardial biopsy-proved myocarditis (9 patients) or borderline myocarditis (11 patients) were studied to determine whether these 2 histologic subsets of patients with inflammatory heart disease differed in their response to a 6- to 8-week course of immunosuppressive therapy. All patients received a regimen of prednisone, 1.0 mg/kg/day, and azathioprine, 1.5 mg/kg/day, followed by repeat endomyocardial biopsy and reevaluation of LV function. LV function improved significantly in the group with borderline myocarditis, as assessed by LV stroke work--end-diastolic volume ratio (0.26 +/- 0.17 to 0.54 +/- 0.31 kg.m.ml-1, p less than 0.02), heart rate corrected velocity of circumferential shortening (0.49 +/- 0.30 to 0.80 +/- 0.29 circ.s-1, p less than 0.05), and LV ejection fraction (0.30 +/- 0.15 to 0.47 +/- 0.13, p less than 0.05). LV end-diastolic and end-systolic volume indexes also decreased significantly from 129 +/- 40 to 94 +/- 38 (p less than 0.05) and 90 +/- 37 to 49 +/- 26 ml (p less than 0.02), respectively. No significant change in these indexes of LV function or volume occurred in the myocarditis group. Whereas salutory improvements in cardiac output and filling pressures were found in both groups, objective improvement in LV function assessed by complementary indexes of contractility was greatest in the borderline myocarditis group. It is concluded that short-term immunosuppressive therapy improves LV contractile function and appears to be associated with regression of ventricular dilatation in patients with borderline myocarditis to a greater extent than patients with myocarditis.(ABSTRACT TRUNCATED AT 250 WORDS)