PurposeTo evaluate the effect of a school-based intervention designed to increase physical activity among sedentary adolescent females.MethodsSedentary adolescent females were assigned to a control (n = 22) or intervention (n = 25) group based on school attended. Students at the intervention school enrolled in a special physical education class. All participants completed clinical and behavioral assessments at baseline and after 4 months. Physiological tests included cardiovascular fitness (VO(2) peak via bicycle ergometer), body composition (via dual x-ray absorptiometer; DEXA), and body mass index (BMI). Psychosocial assessments included a physical activity recall, report of lifestyle activity, and self-efficacy, perceived barriers, social support, and enjoyment related to physical activity. Data were analyzed using repeated measures analyses of variance (ANOVAs) for continuous variables, and logistic regression for hard activity (scored as a dichotomous variable).ResultsThe intervention had a significant effect on cardiovascular fitness (p =.017), lifestyle activity (p =.005), and light (p =.023), moderate (p =.007), and hard (p =.006) activity. All changes were in a direction that favored the intervention. There was no effect of the intervention on psychosocial factors related to exercise.ConclusionsA school-based intervention targeting sedentary adolescent females can increase physical activity and prevent a decline in cardiovascular fitness. Moreover, the effect of the intervention generalized to lifestyle activity