Safety, tolerability and efficacy profiles of interferon beta (IFNbeta) therapy in relapsing multiple sclerosis (MS) has been widely verified both in trial settings and in daily clinical practice. However, for a variable percentage of treated patients, it remains only partially effective. In this study, we reported the post-marketing experience of the efficacy of IFNbeta therapy for a large cohort of MS patients regularly attending the MS Outpatient Clinic of "La Sapienza University" in Rome. In this cohort we also sought clinical and paraclinical variables responsible for the clinical course of MS during IFNbeta therapy. Patients that received treatment with one of the IFNbeta formulations for at least 1 year were included. Clinical outcomes (i. e., relapses and disability score) were monitored throughout the entire study period. Magnetic resonance imaging (MRI) scans were performed twice for each subject: at baseline and after 1 year of therapy. The occurrence of more than one relapse during the study period or a sustained disability progression in the Expanded Disability Status Scale (EDSS) score were considered as criteria for the definition of suboptimal clinical response to IFNbeta therapy. During IFNbeta therapy (number of patients 242, mean length of treatment 4.3+/-2.3 years) a reduction in the annualised relapse rate of 59% (p<0.001) was observed. Eighty-six patients (35%) fulfilled the criterion for defining "suboptimal responder" on the basis of relapses, and 69 (28.5%) did the same on the basis of EDSS sustained progression. Twenty-seven (11.1%) patients showed both an EDSS progression and two or more relapses. The presence of T1-enhancing lesions and new T2 hyperintense lesions on the scan performed after the first year of therapy were the best MRI features associated with both the occurrence of relapses during the treatment period (OR for enhancing lesions and relapses 3.6; OR for new T2 lesion and relapses 2.8). The present post-marketing experience confirms the efficacy of IFNbeta in modifying the natural course of MS and encourages the use of paraclinical variables measuring subclinical disease activity as surrogate markers to monitor the clinical course of MS during IFNbeta therapy.