The value of a short course of intensive immunosuppression with cyclophosphamide in stabilising chronic progressive multiple sclerosis (MS) was examined in a randomised single-blinded, placebo-controlled clinical trial. Forty two patients, from the Kaiser Permanente Medical Care Program, Northern California, were studied. Twenty two patients received a short course of cyclophosphamide in an outpatient neurology clinic until their leucocyte counts fell below 4000/mm3, and 20 patients received folic acid. Level of disability, impairment of functional systems, and performance of social roles were assessed before randomisation and reassessed 12, 18, and 24 months after therapy. In both the cyclophosphamide and folic acid groups, the mean level of disability increased from the baseline examination to the 12 month follow up examination (the primary endpoint) by 0.5 on Kurtzke's Expanded Disability Status Scale, indicating similar disease progression in the two groups. Although immunosuppression therapy can be safely administered to MS patients in an outpatient clinic, evidence of substantial benefits was not found.