Sotiropoulos, A, Travlos, AK, Gissis, I, Souglis, AG, and Grezios, A. The effect of a 4-week training regimen on body fat and aerobic capacity of professional soccer players during the transition period. J Strength Cond Res 23(6): 1697-1703, 2009-The purpose of this study was to evaluate the changes in body fat percentage and aerobic capacity in professional soccer players, after the implementation of a specific 4-week training regimen during the transition period. Fifty-eight professional soccer players of the Greek Premier National Division were separated in experimental (n = 38) and control groups (n = 20). Body composition and maximum oxygen intake were evaluated before and after a 4-week training regimen followed during the transition period. The experimental design used for analyzing weight (kg), percent body fat (%) and V̇O2 max values (ml·kg−1·min−1) was a 2 × 2 (Groups × Measures), with Groups as a between-subjects factor and Measures as a within-subjects factor. The level of significance was set at p ≤ 0.05 for all analyses. Analyses of variances showed that the experimental and the control groups achieved statistically significant (a) increases from pretest to posttest measures in body weight (0.595 kg and 1.425 kg, respectively) and percent body fat (0.25 and 0.82, respectively), and (b) decreases in V̇O2max values from pretest to posttest measures (0.81 and 3.56, respectively). The findings of the study revealed that the players who followed the training regimen compared with the players that did not follow any specific training program gained less weight and body fat and exhibited lower reduction in their V̇O2 max values.