In the prevention of relapses and disability by interferon subcutaneously in multiple sclerosis (PRISMS) study, 560 patients with relapsing-remitting multiple sclerosis were randomized to receive subcutaneous interferon (IFN) beta-1a, 22 or 44 mug three times weekly, or placebo, for 2 years. Patients receiving placebo were then re-randomized to one of the two doses of IFN beta-1a for a further 2 years, whilst patients receiving active treatment continued their original treatment. Safety assessments were performed throughout the study. The most common adverse events for patients originally randomized to active treatment were injection-site inflammation (72% of patients had at least one event), headache (71%) and influenza-like symptoms (69%). These were generally mild in nature and most frequent during the first month of treatment. The 4-year adverse event profiles for the two IFN beta-1a doses were comparable with those observed during the initial phase of the study and, for the most part, with each other. There was no association between IFN beta-1a and depression or suicide/attempted suicide. The most common laboratory abnormalities were asymptomatic lymphopenia and elevated serum liver transaminase levels. These were generally mild and resolved spontaneously. Therapy with subcutaneous IFN beta-1a three times weekly for up to 4 years was well tolerated without dose-limiting safety concerns.