The objective was to evaluate the safety, tolerability and effectiveness of intramuscular (IM) interferon beta-1a (IFNbeta-1a; Avonex, Biogen) 30 mg once a week in patients with onset of symptoms of multiple sclerosis (MS) in childhood or adolescence. Patients with a diagnosis of definite MS according to McDonald's criteria, relapsing course according to Lublin's criteria, onset of symptoms of MS before 16 years of age, and who had received IM IFNbeta-1a therapy before 16 years of age were eligible for the study if they had a pretreatment and treatment duration of at least 6 months. Clinical and laboratory evaluations were performed every 3 months. A total of 52 patients were identified as receiving treatment with IM IFNbeta-1a 30 mg once a week before 16 years of age. Mean age at onset of symptoms of MS was 11.7+/-2.7 years, mean disease duration was 25.9+/-30.3 months, mean annualised relapse rate was 1.9+/-1.1 and mean Expanded Disability Status Scale (EDSS) score was 1.5+/-1.1. After a mean (+/-SD) treatment duration of 42.9+/-19.9 months, annualised relapse rate decreased to 0.4+/-0.5. Final EDSS score was 1.3+/-1.1. Adverse events were recorded for 35 (67%) patients (flulike syndrome, 33%; headache, 29%; myalgia, 21%; fever, 11%; fatigue, 6%; nausea and vomiting, 6%; and skin reaction, 4%); most were transient. IM IFNbeta-1a was effective and well tolerated in these paediatric patients with MS.