BRFSS is a population-based, random-digit–dialed telephone survey of the civilian, noninstitutionalized U.S. population aged ≥18 years. Forty-three states and the District of Columbia collected data about physical activity for 1990, 1991, 1992, 1994, 1996, and 1998. Data were not collected by all states during 1993, 1995, and 1997. Respondents were asked about the two physical activities or exercises they engage in most often and about the frequency, duration, and distance (as appropriate) of each activity. Responses were then classified as one of 56 selected activities. Moderate activity was defined as any of the 56 selected activities, and vigorous activity was defined as aerobic physical activity classified as vigorous-intensity based on estimated metabolic expenditure (MET). To classify an activity as vigorous, it must be aerobic with an assigned MET value3 that is at least 60% of a person's maximal cardiorespiratory capacity (MCC). MET values are determined using two regression equations for MCC4: one for men (METS 60%MCC = [0.6 × (60—0.55 × age)]/ 3.5) and one for women (METS 60%MCC = [0.6 × (48—0.37 × age)]/3.5).