Purpose: To test the hypothesis that sex-related differences in exercise behavior during adolescence are confounded by biological age. Methods: Biological age and leisure-time exercise activity were assessed in 103 male and 83 female British Year 9 students (mean = 14.04 years, SD = 0.31). To assess exercise behavior, participants completed the Leisure-Time Exercise Questionnaire (LTEQ). Percentage of predicted adult height attained at measurement was used as an estimate of biological maturity status. Males reported more frequent engagement in strenuous forms of exercise and total exercise activity than females. Results: Sex differences in exercise behavior were attenuated and nonsignificant when biological maturity was controlled for. Conclusions: Sex-related differences in biological maturity contribute to sex-related differences in exercise behavior during adolescence. (C) 2008 Society for Adolescent Medicine. All rights reserved.