The aims of the study were (1) to assess the relationships among leisure-time physical activity, sedentary behaviors, and measures of health-related and performance-related physical fitness, and (2) to determine the primary predictors of performance-related physical fitness from the variables investigated. This study updates the literature with the inclusion of Internet use and draws attention to other possible ways in which leisure-time physical activity and physical fitness may be operationally defined. In this investigation, involving 129 high school girls, television-watching and Internet-use time were evaluated for one week, and their relationships to various physical fitness parameters (e.g., BMI, percent fat, cardiorespiratory fitness) were assessed. Leisure-time physical activity, quantified by a 21-day physical activity diary and a compendium of physical activities with MET intensities (Ainsworth et al., 2000), were compared to the aforementioned variables. The study found no significant correlations of television-watching time or Internet-use time with BMI, one-mile-run time, and three different operational definitions of leisure-time physical activity (in minutes per week, absolute caloric expenditure derived from that activity, and caloric expenditure per kilogram of body weight derived from that activity). The authors concluded that leisure-time physical activity is significantly (and inversely) associated with physical fitness only when expressed by an actual performance measure, but not when represented by the sum of the skinfold measurements, body fat, or BMI. Physical fitness is unrelated to Internet-use time or television-watching time. Furthermore, since these sedentary behaviors are unrelated to leisure-time physical activity, yet positively correlated with each other, it is suggested that time spent watching television or on the Internet does not occur at the expense of leisure-time physical activity.