Shortly after the March 28, 1979, accident at the Three Mile Island (TMI) nuclear plant outside Harrisburg, Pa., the Pennsylvania Department of Health, in conjunction with the Centers for Disease Control and the U.S. Bureau of the Census, conducted a census of the 35,930 persons residing within 5 miles of the plant. With the help of 150 enumerators, demographic and health-related information was collected on each person to provide baseline data for future short- and long-term epidemiologic studies of the effects of the accident. Individual radiation doses were estimated on the basis of residential location and the amount of time each person spent in the 5-mile area during the 10 days after the accident. Health and behavioral resurveys of the population will be conducted approximately every 5 years. Population-mobility, morbidity, and mortality will be studied yearly by matching the TMI Population Registry with postal records, cancer registry records, and death certificate data. Because the radiation dose from TMI was extremely small, any increase in morbidity or mortality attributable to the accident would be so small as not to be measurable by present methods; however, adverse health effects as a result of psychological stress may occur. Also, a temporary increase in reporting of disease could occur because of increased surveillance and attention to health.