Estimates the effects of various prototypical health insurance options on demand for medical services. The data indicate that under full coverage or 25 percent maximum coinsurance, demand for hospital services would rise modestly. However, either program would greatly increase demand for ambulatory services and would stress the delivery system, with resulting increased price of physicians' services, queueing, or less physician time per patient — all without increasing total delivery of ambulatory services. Ambulatory services would be redistributed from the affluent to the poor. A catastrophic health insurance program would not stress the ambulatory system. Reorganization of the delivery of ambulatory services into prepaid groups would probably not increase productivity, nor would emphasis on preventive medicine reduce overall demand for health services. National insurance providing more health services would not appreciably affect objective indexes of health (life expectancy), but should improve subjective but unquantifiable elements such as quality of life.