OBJECTIVE: To determine whether giving interferon-alpha or thymomodulin in addition to conventional treatment improves cardiac function in patients with idiopathic myocarditis and idiopathic dilated cardiomyopathy. DESIGN: Single-centre, randomised, open label, parallel group comparison of conventional treatment plus interferon-alpha, conventional treatment plus thymomodulin, and conventional treatment alone. PATIENTS: 38 patients aged 19-54 years (23 men) with biopsy-proven myocarditis or dilated cardiomyopathy. 12 were treated with conventional treatment alone, 13 were treated with interferon-alpha and conventional treatment, and 13 with thymomodulin and conventional treatment. SETTING: Tertiary cardiac referral centre. MAIN OUTCOME MEASURES: Clinical evaluation, echocardiography, and Holter monitoring at baseline, 6 months, and 1 and 2 years. Radionuclide ventriculography at rest and during exercise after 2 years. Endomyocardial biopsy at baseline and after a year if the initial diagnosis was myocarditis. RESULTS: Left ventricular ejection fraction was improved in 21 (81%) of 26 patients after interferon-alpha or thymomodulin administration and in 8 (66%) of 12 conventionally treated patients (P < 0.05) at 2 year follow up. The maximum exercise time was significantly longer at 2-year follow up in patients treated with immunomodulators (mean (SEM) 5.1 (0.6) minutes for interferon-alpha and 5.0 (0.4) minutes for thymomodulin) than in conventionally treated patients (3.3 (0.4) minutes). Left ventricular ejection fraction during exercise (assessed by radionuclide ventriculography) improved in 9 of 12 patients treated with interferon-alpha, 10 of 12 patients treated with thymomodulin, and 3 of 9 conventionally treated patients at 2 year follow up. The electrocardiogram was normal in 21 (88%) of 24 patients after interferon-alpha or thymomodulin treatment and 2 (22%) of 9 conventionally treated patients. At 2 year follow up, 19 (73%) of 26 patients treated with immunomodulators and 4 (25%) of 12 conventionally treated patients had improved their functional class. CONCLUSIONS: The results suggest that treatment of idiopathic myocarditis and/or idiopathic dilated cardiomyopathy with interferon-alpha or thymomodulin induces an earlier and significantly superior clinical improvement than conventional treatment alone.