Research from several countries has documented a decline in physical activity (PA) levels and in participation in organized sport with increasing age, indicating that organized sport may be of importance to adolescents' cardiorespiratory fitness (CRF). Purpose: The purpose of this study was to examine how regular participation in organized and unorganized PA affected the development of adolescents' CRF (peak oxygen consumption [VO[subscript 2]peak]), when controlled for sex interaction. Method: Data on direct measures of VO[subscript 2]peak and participation in organized PA among adolescents organized into 3 groups (participation in organized sport, participation in unorganized PA, and no weekly PA) were collected from 76 students (39 boys and 37 girls), when they were aged 14 and 19 years old. Results: Statistically significant differences were found between VO[subscript 2]peak values in the 3 groups at both 14 years of age, F(2, 73) = 7.16, p < 0.05, n[superscript 2] = 0.170, and 19 years of age, F(2, 73) = 14.00, p < 0.05, n[superscript 2] = 0.300, independent of sex at both 14 and 19 years of age, F(2, 73) = 0.05, p > 0.05, n[superscript 2] = 0.02, and F(2, 73) = 0.05, p > 0.05, n[superscript 2] = 0.00. Adolescents participating in organized sport also had statistically significantly higher VO[subscript 2]peak values than adolescents participating in unorganized PA and those with no weekly PA, at both 14 and 19 years of age. Conclusion: From a health perspective, in terms of CRF, the findings highlight the importance of encouraging adolescents to participate in organized sport and to refrain from dropping out of organized sport programs.