The decision to use interferon beta (IFN-b) as a treatment for relapsing-remitting multiple sclerosis (RRMS) is based on both clinical characteristics and course of the disease. To better identify the profile of responders, the relationships between baseline clinical/MRI characteristics and therapeutical response was analyzed in 49 patients with RRMS randomly assigned to receive subcutaneously 3 or 9 MIU of IFN-b-1a. The therapeutical response was evaluated as a per cent change in the mean number and volume of monthly Gd-enhancing lesions in both first (early response) and second (late response) 6-month period of treatment, compared to the 6-month pre-treatment period. A better early response was seen in patients with a lower number of relapses during the pre-treatment period, while the late response was favourably influenced by a lower baseline EDSS and the high dose. Our findings suggest that the effect of IFN-b-1a on disease MRI activity is dose-related and dependent on the relapse rate and the level of disability before treatment.