Previous studies reported that a 2- to 3-week course of IV cyclophosphamide plus adrenocorticotropic hormone (ACTH) induction can temporarily halt progressive MS for a period of 12 months in the majority of patients treated, after which reprogression occurs. The Northeast Cooperative Multiple Sclerosis Treatment Group was formed to determine whether outpatient pulse cyclophosphamide therapy could affect reprogression and whether there were differences between a modified induction regimen and the previously published regimen. Two hundred fifty-six progressive MS patients were randomized into four groups to receive IV cyclophosphamide/ACTH via the previously published versus a modified induction regimen, with or without outpatient IV cyclophosphamide boosters (700 mg/m2 every other month for 2 years). There were blinded evaluations performed every 6 months. Results demonstrate that (1) there were no differences between the modified and the published induction regimens either in terms of initial stabilization or subsequent progression; (2) without boosters, the majority of patients continued to progress; and (3) in patients receiving boosters, there was a statistically significant benefit at 24 months and 30 months (p = 0.04). Time to treatment failure after 1 year was also significantly prolonged in the booster versus the nonbooster group (p = 0.03). Age was the most important variable that correlated with response to therapy in that amelioration of disease progression occurred primarily in patients 40 years of age or younger. Boosters had a significant benefit on time to treatment failure in patients ages 18 to 40, p = 0.003, but not in patients ages 41 to 55, p = 0.97.(ABSTRACT TRUNCATED AT 250 WORDS)