Twenty relapsing-remitting (RR) clinically definite MS patients were treated with 9 MIU intramuscular recombinant interferon alpha-2a (rIFNA) (Roferon-A, Roche) (n = 12) or placebo (n = 8) every other day for 6 months and followed up for a further 6 months after stopping treatment. Numbers of active lesions at MRI and of patients with clinical-MRI signs of disease activity and lymphocyte interferon gamma production, which were decreased during treatment, returned to values similar to baseline and placebos after stopping treatment. rIFNA chronic therapy seems therefore needed in order to maintain drug efficacy. Side effect profile was monitored, too, for over 1 year in the same 20 patients plus 25 additional RR MS patients. Besides the typical side effects of type I interferon therapy (fever, fatigue, depression, lymphopenia, hepatic enzyme elevation), occurrence of serum autoAbs was noted in 30% patients (in 60% antinuclear and in 80% antithyroid autoAbs). In two patients rIFNA treatment was stopped, in one case for antithyroid autoAbs and hypothyroidism, in the other for antinuclear autoAbs and a five-fold increase of ALT. A careful monitoring of serum autoAbs and of signs of thyroid or liver damage must always precede and accompany longterm type I IFN therapy.