The 2-year outcomes of three non-parallel, prospective, placebo-controlled studies (BENEFIT, CHAMPS, ETOMS) have shown that IFN beta-1b s.c. (Betaferon®), IFN beta-1a i.m. (Avonex®) and IFN beta-1a s.c. (Rebif®) similarly prevented conversion of clinically isolated syndromes (CIS) into clinically definite multiple sclerosis (CDMS). Smaller proportions of CIS patients in the Betaferon®, Avonex® and Rebif® groups converted to CDMS as compared with fractions in the placebo groups (28%, 21%, 34% vs. 45%, 35%, 45%); p from 0.047 to 0.00007. The mean number of days to CDMS development was prolonged by three products of IFN beta (363, 375, 569 days), but not by placebo (255, 309, 252 days); p between <0.05 and 0.00007. The mean or median number of new T2-related brain lesions was more strongly reduced in all studies by the verum than by placebo (1.0 vs. 2.0, 2.1 vs. 5.0, 2.0 vs. 3.0); the differences were significant. The mean or median number of new, enhancing lesions was also significantly reduced in the BENEFIT and in the CHAMPS studies, but not in the ETOMS study. An indirect comparison proved that three products of IFN beta partially prevented or delayed conversion of CIS into CDMS and hampered the evolution of new brain lesions on magnetic resonance images.