The aim of this prospective study is to examine the relation of physical activity and cardiorespiratory fitness to health in the middle and later years of life. Health is broadly defined here and encompasses measures of cause-specific and all-cause mortality, nonfatal diseases, and functional health status (FHS). The study sample consists of 10,809 women and 36,764 men who have been or will be examined at least once at a preventive medicine clinic during the interval from 1970 to 1998. Total follow-up will reach approximately 137,091 woman-years and 500,403 man-years by 1998, which should yield 2,265 deaths. An important strength of the study is the defined sample, for each member of which there is an objective measure of cardiorespiratory fitness from a maximal exercise test. In addition, the database includes demographic, behavioral, and clinical data from the examination, health habit and health status measurements from mail surveys, and mortality information. The fitness and clinical data offer advantages over studies in which only self-reports of physical activity and clinical status are available. This extensive database permits the provision of controls for baseline health status and numerous demographic, behavioral, health history, and clinical variables. There are plans to add to the database by extending mortality follow-up for 4 years, and by conducting another mail-back survey to monitor nonfatal endpoints. The expanded database will allow for analyzes to address current unanswered questions in exercise epidemiology. Although there is general agreement that regular exercise contributes to good health and function, further evaluation of several details can still provide important results. The specific relationship of exercise intensity to health outcomes is unclear. Although current public health statements emphasize the value of moderate intensity exercise, some experts think that higher intensity activity should be recommended. The planned study includes data on speed and amount of walking or jogging on 8,496 women and 29,926 men; it is expected that these data will allow for a more specific evaluation of exercise intensity than has been possible in previous studies. The cohort, as described, is the largest prospective study of women who have had a maximal exercise test at baseline. This allows for analyzes on fitness and cause-specific morbidity and mortality, change in fitness and health, and the relation of fitness to FHS in women. It is expected that by 1998, 36,414 woman-years and 129,094 man-years of follow-up will be completed on persons >60 years of age, which makes this cohort increasingly valuable for the evaluation of the role of physical activity and cardiorespiratory fitness in the preservation of health and function and promotion of a high quality of life as people age. The investigators state that they expect that extension of this study will have the potential to make important additional contributions to understanding the health benefits of a fit and active way of life.