After approval by the European Union in 1996 of interferon-beta for treatment of relapsing-remitting multiple sclerosis (RRMS), 862 patients in Denmark have received treatment (by April 1999), and 304 of those were enrolled into an open labelled randomised trial to compare the efficacy and tolerability of treatment with interferon-beta-1b (Betaferon) 8 MIU subcutaneous every other day and interferon-beta-1a (Rebif) 6 MIU subcutaneous once a week. Primary and secondary end-points included: (1) the relapse rate, (2) tolerability, (3) neutralizing antibody formation using the same specialised laboratory and validated assay, (4) time to first relapse, (5) time to sustained progression, and (6) number of gadolinium-enhancing lesions in T1-weighted MRI and the total lesion load in T2-weighted MRI. All the records are kept in a nationwide clinical database connected with the Danish Multiple Sclerosis Registry. The randomised trial will be completed by the end of 1999, and the results are not yet available. The annual relapse rate in all treated patients until April 1999 was 0.73, the annual rate of steroid treated relapses was 0.21. By 1 April 1999 8.8% of the patients had discontinued treatment and 10.4% had changed to another interferon-beta preparation. It has been planned to extend the follow-up of all patients treated according to the Danish National Protocol for a period of several years. The continuous and nation-wide registration of the course of the disease in interferon-beta treated RRMS patients may provide valuable knowledge of the treatment-modified long-term course of the disease in a cohort of patients, selected on the basis of well-defined criteria.