Recovery of various components of the immune system was followed in eight patients with multiple sclerosis who had received monthly pulses of cyclophosphamide (CY) for approximately one year. CD8 cell numbers and NK and ADCC functions recovered in 1-2 months; B cells and FcR+ cell numbers recovered in 2-4 months. The recovery of CD4 cells and total T cell numbers, CD4/CD8 ratio and proliferative responses to PHA took more than 4 months. The impaired proliferation was not attributable to low IL-2 receptor expression. Once immunosuppression has been achieved pulse administration of CY at 2- to 4-month intervals may be feasible for long-term maintenance treatment.