Prospective clinical open label follow-up of 52 multiple sclerosis patients treated with interferon beta-1b. After 18 months of treatment at standard 8 million international units (MIU) dose, subcutaneously on alternate days, IFNB dose was increased to 12 MIU in ten clinically non-responder patients. Eighteen months after, mean exacerbation rate, number and severity of exacerbations and number of patients with exacerbations or requiring corticosteroid treatment significantly improved, becoming similar to those of IFNB responders, always treated with 8 MIU. Baseline EDSS score of non-responders was higher than that of responders. Frequency and severity of adverse events were trending higher and dropout frequency higher in 12 MIU IFNB-treated patients.