There is no therapeutic agent proven to alter the outcome of ARF. This prospective, double-blind, randomized, placebo-controlled trial evaluated IVIG, a proven immunomodulator, in patients with a first episode of ARF. Patients were stratified by presence and severity of carditis prior to randomization. 61 patients (34 male, aged 6.2-15.9 years) were randomly allocated to receive lg/kg IVIG on day 1 and 2, and 0.4g/kg on day 14 and 28, or a placebo infusion. Clinical, laboratory and echocardiographic evaluation was performed at 0, 2, 4, 6, 26, and 52 weeks. The majority of patients presented with clinical carditis (38) and/or migratory polyarthritis (39). There was no difference between groups in duration of arthritis (2.9 vs 3.7 days), length of time receiving aspirin (17 vs 20 days), or normalisation of ESR and acute phase proteins at 6 weeks. By echocardiography, 67% in the IVIG group and 69% in the placebo group had carditis at baseline. There was no significant difference in cardiac outcome at 2, 4, 6, and 26 and 52 weeks. At 1 year, clinical and echocardiographic carditis was present in 26 and 41% of those receiving IVIG, compared with 31 and 50% of those receiving placebo, respectively. There was also no difference between groups in the proportion of valves with valvulitis, or the severity of mitral or aortic regurgitation at 1 year. IVIG does not alter the natural history of ARF, with no significant difference in the rate of improvement of clinical, laboratory or echocardiographic parameters.