Background: Intravenous immunoglobulin (IVIg) has been reported to reduce dise ase activity in patients with relapsingremitting multiple sclerosis. We assessed the effect of IVIg treatment in patients after the first neurological event sug gestive of demyelinative disease and evaluated the occurrence of a second attack and dissemination in time demonstrated by brain magnetic resonance imaging with in the first year from onset. Methods: We conducted a randomized, placebo con trolled, double blind study in 91 eligible patients enrolled within the first 6 weeks of neurological symptoms. Patients were randomly assigned to receive IVIg treatment (2 g/kg loading dose) or placebo, with boosters (0.4 g/kg) given onc e every 6 weeks for 1 year. Neurological and clinical assessments were done ever y 3 months, and brain magnetic resonance imaging was performed at baseline and t he end of the study. Results: The cumulative probability of developing clinicall y definite multiple sclerosis was significantly lower in the IVIg treatment grou p compared with the placebo group (rate ratio, 0.36 [95%confidence interval, 0 . 15 0.88]; P = .03). Patients in the IVIg treatment group had a significant red u ction in the volume and number of T2 weighted lesions and in the volume of gado lin ium enhancing lesions as compared with the placebo group (P = .01, P = .01 , and P = .03, respectively). Treatment was well tolerated, compliance was high, and incidence of adverse effects did not differ significantly between groups. C onclusions: Intravenous immunoglobulin treatment for the first year from onset o f the first neurological event suggestive of demyelinative disease significantly lowers the incidence of a second attack and reduces disease activity as measure d by brain magnetic resonance imaging. Background: Intravenous immunoglobulin (IVIg) has been reported to reduce diseseactivity in patients with relapsingremitting multiple sclerosis. We assessed the effect of IVIg treatment in patients after the first neurological event sug gestive of demyelinative disease and evaluated the occurrence of a second attack and dissemination in time demonstrated by brain magnetic resonance imaging with in the first year from onset. patients were randomlyized to the placebo con trolled, double blind study in 91 eligible patients enrolled within the first 6 weeks of neurological symptoms. Patients were randomly assigned to Neurological and clinical assessments were done ever y 3 months, and brain magnetic resonance imaging was performed (2 g / kg loading dose) or placebo, with boosters (0.4 g / kg) given on c e every 6 weeks for 1 year at baseline and t he end of the study. Results: The cumulative probability of developing clinical l y definite multiple sclerosis was signi ficantly lower in the IVIg treatment grou p compared with the placebo group (rate ratio, 0.36 [95% confidence interval, 0. 15 0.88]; P = .03). Patients in the IVIg treatment group had a significant red ution in the volume and number of T2 weighted lesions and in the volume of gado lin ium enhancing lesions as compared with the placebo group (P = .01, P = .01, and P = .03, respectively). Treatment was well tolerated, compliance was high, and incidence of adverse effects did not differ significantly from groups. C onclusions: Intravenous immunoglobulin treatment for the first year from onset of the first neurological event suggestive of demyelinative disease significantly lowers the incidence of a second attack and reduces disease activity as measure d by brain magnetic resonance imaging.