Comprehensive data on the costs of multiple sclerosis is sparse. We conducted a survey of 606 persons with MS who were members of the National Multiple Sclerosis Society to obtain data on their cost of personal health services, other services, equipment, and earnings. Compensation of such cost in the form of health insurance, income support, and other subsidies was measured. Survey data and data from several secondary sources was used to measure costs incurred by comparable persons without MS. Based on the 1994 data, the annual cost of MS was estimated at over $34 000 per person, translating into a conservative estimate of national annual cost of $6.8 billion, and a total lifetime cost per case of $2.2 million. Major components of cost were earnings loss and informal care. Virtually all persons with MS had health insurance, mostly Medicare/Medicaid. Health insurance covered 51 per cent of costs for services, excluding informal care. On average, compensation for earnings loss was 27 per cent. MS is very costly to the individual, health care system, and society. Much of the cost (57 per cent) is in the form of burdens other than personal health care, including earnings loss, equipment and alternations, and formal and informal care. These costs often are not calculated.