In patients with relapsing-remitting multiple sclerosis (MS), IVIG was shown to reduce the relapse rate and progression of disability. In patients with chronic progressive MS, a beneficial effect of IVIG was not documented in placebo controlled studies. This trial investigated the influence of IVIG in primary (PPMS) and secondary (SPMS) chronic progressive MS. Two-hundred and thirty-one patients stratified for PPMS (n=34) and SPMS (n=197) were randomly assigned to IVIG 0.4 g/kg per month or to placebo for 24 months. Primary endpoints were 1) the time to sustained progression of disease identified as worsening of the expanded disability status scale (EDSS) sustained for 3 months, and 2) the improvement of neurological functions defined by a patient's best EDSS score. Secondary endpoints were the proportion of patients with sustained progression, the relapse rate, the assessment of fine motor skills, visual evoked potentials, contrast sensitivity, depression and quality of life. Analysis of the intention-to-treat (ITT) population of combined PPMS and SPMS patients showed that the mean time to sustained progression was 74 weeks in the IVIG compared with 62 weeks in the placebo group (P=0.0406). When PPMS and SPMS patients were analysed separately, the time to sustained progression was also longer in the IVIG group, but the difference was not significant. There was no IVIG-mediated improvement in neurological functions. In the combined per protocol (PP) treated patients, IVIG treatment prolonged time to sustained progression by 13 weeks (P=0.0396). PPMS patients, but not SPMS patients showed a slight favourable IVIG effect on the best EDSS score. In the combined ITT population there were less patients with sustained progression in the IVIG than in the placebo group (P=0.028). The difference was significant in PPMS (P=0.016), but not in SPMS patients. In the combined PP population, there was a trend for a favorable IVIG effect on the rates of patients with sustained progression. In patients with PPMS, this IVIG effect reached significance (P=0.036). Other secondary endpoints did not show significant differences between treatment groups. Eighteen patients with PPMS and 102 patients with SPMS withdrew from the study for various reasons. Treatment was generally well tolerated. It was concluded that monthly IVIG infusion could delay progression of disease in patients with PPMS, and that there was a trend in favour of IVIG treatment in patients with SPMS.