This paper reviews the association of cardiorespiratory fitness to all-cause and cardiovascular disease mortality. Data are presented from the Aerobics Center Longitudinal Study (ACLS), which is a prospective study of 25,341 men and 7080 women. These individuals completed a health screening examination and were followed for more than 8 years for mortality. Low cardiorespiratory fitness was one of the strongest predictors of mortality in this study, with the risk of low fitness being comparable to smoking. Risk of low fitness persisted after adjustment for a long list of other risk factors and other potential confounders. Previously unpublished data on metabolic and cardiovascular responses to the exercise test used in the ACLS are also provided. This additional information further characterizes this cohort, and gives additional data and methods that can be used by others for risk assessment and stratification.