An increasing number of evidence is showing that during prolonged treatment of relapsing-remitting multiple sclerosis (RRMS) with interferon (IFN) beta 1a or IFN beta 1b, the patients may develop serum anti-IFN antibody. It has been argued that some of the RRMS patients receiving IFN beta, who developed antibodies to IFN, lose them over time even though the treatment continues. To gain further insights into this issue, we performed a study to establish what happened to binding antibodies (BAB) and neutralizing antibodies (NAB) in 42 RRMS patients treated for 6 years with IFN beta 1a and/or IFN beta 1b. While the data of BAB analysis did not allow to reach definite conclusions, the results on NAB development confirm that the presence of this type of antibodies is transitory; in fact, most of the positive patients reverted to seronegative, although the IFN treatment is still ongoing; the only patients who were positive for NAB at 6 years of treatment are those whose serum contains high concentration of them. The paper also shows that patients lose antibodies to IFN independently on the type of IFN used for the treatment. In conclusion, the data indicate that the disappearance of the anti-IFN antibodies from the serum while the patients are still undergoing IFN treatment depends on the titer of antibodies but not on the type of IFN administered.