Background: The role of the immune system in multiple sclerosis (MS) has been proved. Pentoxifylline has an inhibitory effect on phosphodiesterase and the cytokine products. In some studies therapeutic effects of pentoxifylline have been documented in MS patients. Objectives: The aim of our study was to investigate the effects of pentoxifylline on the recurrence and the disease course in MS patients. Patients and Methods:In our double-blinded clinical trial study, 44 newly diagnosed relapsing-remitting MS patients were studied for 12 months. They were divided into 2 groups: in group 1, 22 patients received interferon and oral pentoxifylline (800 mg daily for 2 months) and in group 2 (control group), 22 patients received interferon with a placebo. The monthly attack rates and the clinical condition of the patients, using the expanded disability status scale (EDSS), were measured in 3 different times along the study (beginning of the treatment, 6 months after and at the end of the treatment). Analysis of the data was done using t-test and the nonparametric Mann-Whitney U test. Results: There was not any significant difference between the 2 groups in terms of age and sex. The recurrence in the first year was 0.4% and 0 in group 1 and 2, respectively. Also there was not any difference in the clinical course and the recurrence of the disease between two groups in one year of follow-up. Conclusions: Adding pentoxifylline to interferon does not have any synergistic therapeutic effects on the reduction of relapse frequency and EDSS in patients with relapsing-remitting multiple sclerosis in comparison with interferon.