In a survey of disease course, the efficacy and tolerability of 24-month interferon beta-1b therapy for relapsing remitting multiple sclerosis (RRMS) were evaluated in 410 patients. The investigation aimed at obtaining data from general practice and of possibly unknown, unexpected adverse reactions. In the 241 patients still on therapy, efficacy was rated after 24 months as "good" or "very good" in 75% of cases. After 24 months, 36.9% of the patients had no exacerbation (baseline 0.3%). Annual exacerbation rates dropped from 1.5 before treatment to 0.7 in the second treatment year. In the 2 years before treatment, 66.2% had worsened by at least 0.5 points on the extended disability status scale (EDSS). This proportion was reduced to 41.2% after 2 years of treatment. The safety profile corresponded to results from controlled trials. This postmarketing survey supports data from the published controlled interferon beta-1b studies and confirms the main effects of this therapy under routine conditions in general practice.