We examined immune function and changes in T cell populations over a 1-year period in a series of progressive multiple sclerosis (MS) patients treated with different regimens of cyclophosphamide/ACTH as part of the Northeast Multiple Sclerosis Treatment Group. Our studies were designed to determine the effect of different cyclophosphamide/ACTH regimens on T cell populations and functional immune assays and to determine whether immune measures could be identified to predict which patients responded favorably to treatment. Cyclophosphamide/ACTH infusions significantly decreased the proportion of peripheral blood CD4+ T cells at 2, 6 and 12 months following treatment while there was a tendency for increased CD8 expression. This was associated with significant decreases of CD4/CD8 ratios at 2, 6 and 12 months following treatment compared to pretreatment. No changes in CD3+ T cells were observed while there were increased percentages of CDw26 (Ta1) positive and IL-2 positive T cells following treatment. The only T cell populations predictive of improvement were percentages of either CD3+ or CD4+ cells where increased percentages of either these populations at 2 months following cyclophosphamide/ACTH infusions were associated with improvement at both 6 and 12 months. In terms of functional immune measures, we found that cyclophosphamide/ACTH treatment decreased the level of proliferation in the allogeneic mixed lymphocyte reaction (MLR) at 2 months and of spontaneous proliferation of mononuclear cells at 12 months following therapy. Changes in spontaneous proliferation were predictive of clinical improvement at 12 months in that subjects with improved scores on the disability status scale (DSS) had decreases in spontaneous proliferation at 12 months as compared to pretreatment, whereas those stable or worse did not change significantly. Thus, our studies have demonstrated specific alterations in immune function following immunosuppression with cyclophosphamide/ACTH and suggest that certain immune measures may be linked to a positive clinical response and thus associated with disease progression in MS.