Mitoxantrone (MTX) is an antineoplastic agent approved for treatment of secondary progressive and rapidly worsening relapsing-remitting multiple sclerosis (MS). We designed a longitudinal open-label prospective study to evaluate the efficacy and toxicity of MTX over a 2-year treatment period with a further 3-year follow-up. Fifty consecutive MS patients were included and received MTX intravenously (8 mg/m(2) every 2 months for a total of 12 infusions). Efficacy was assessed clinically and by brain MRI performed before MTX therapy, at the end of treatment and at the end of each year of follow-up. Forty-nine patients completed the 5-year study, 44 (89.8%) completed the MTX course, five (10.2%) interrupted the treatment because of side effects. Fifteen (30.6%) patients showed Expanded Disability Status Scale (EDSS) progression on treatment and nine (18.4%) during follow-up. Seventeen (34.7%) patients had enhancing lesions at baseline, nine (18.4%) at the end of treatment, but none at the end of follow-up. In conclusion, we observed EDSS progression in about 1/3 of the patients during the treatment period and in 1/5 during the further 3-year follow-up period. This evidence suggests a delayed beneficial effect after MTX treatment is completed with only a minority of patients showing disability progression once the drug was suspended.