Abstract: Smith, MM, Sommer, AJ, Starkoff, BE, and Devor, ST. Crossfit-based high-intensity power training improves maximal aerobic fitness and body composition. J Strength Cond Res 27(11): 3159–3172, 2013—The purpose of this study was to examine the effects of a crossfit-based high-intensity power training (HIPT) program on aerobic fitness and body composition. Healthy subjects of both genders (23 men, 20 women) spanning all levels of aerobic fitness and body composition completed 10 weeks of HIPT consisting of lifts such as the squat, deadlift, clean, snatch, and overhead press performed as quickly as possible. Additionally, this crossfit-based HIPT program included skill work for the improvement of traditional Olympic lifts and selected gymnastic exercises. Body fat percentage was estimated using whole-body plethysmography, and maximal aerobic capacity (V[Combining Dot Above]O2max) was measured by analyzing expired gasses during a Bruce protocol maximal graded treadmill test. These variables were measured again after 10 weeks of training and compared for significant changes using a paired t-test. Results showed significant (p < 0.05) improvements of V[Combining Dot Above]O2max in men (43.10 ± 1.40 to 48.96 ± 1.42 ml·kg−1·min−1) and women (35.98 ± 1.60 to 40.22 ± 1.62 ml·kg−1·min−1) and decreased body fat percentage in men (22.2 ± 1.3 to 18.0 ± 1.3) and women (26.6 ± 2.0 to 23.2 ± 2.0). These improvements were significant across all levels of initial fitness. Significant correlations between absolute oxygen consumption and oxygen consumption relative to body weight was found in both men (r = 0.83, p < 0.001) and women (r = 0.94, p < 0.001), indicating that HIPT improved V[Combining Dot Above]O2max scaled to body weight independent of changes to body composition. Our data show that HIPT significantly improves V[Combining Dot Above]O2max and body composition in subjects of both genders across all levels of fitness.