Monthly intravenous cyclophosphamide treatment was compared with placebo treatment for one year in a controlled trial of 14 patients with relapsing/remitting multiple sclerosis. Eight patients received placebo and six patients received cyclophosphamide. The cyclophosphamide group showed a definite trend to have less frequent and less prolonged episodes than the placebo group. When each group served as their own controls, the cyclophosphamide group had a significant decrease in episodes while the placebo group did not. When the placebo group was then given cyclophosphamide, they also had a significant decrease in episodes. When all patients who were receiving cyclophosphamide were combined for analysis, the decrease in episodes was even more evident. Complications were minimal. These results suggest that monthly intravenous doses of cyclophosphamide may influence the frequency and duration of episodes of relapsing/remitting multiple sclerosis.