Several factors contribute to the fact that not all multiple sclerosis (MS) patients respond equally well to long-term interferon beta (IFNbeta) treatment, even if the initial response is adequate. Among these factors, anti-interferon neutralising antibodies (NAbs) may be included. There is increasing evidence that these antibodies have a clinical impact in MS treated patients, which is evident some months following the initiation of treatment with IFN?. Several efforts to reduce the concentration of NAbs, especially when they are in high titres and clinically active, have failed. However, the same efforts may be more effective if applied following early detection of the antibodies, thus leading to a continuation of the initially selected interferon treatment.