Pérez-Castilla, A and García-Ramos, A. Evaluation of the most reliable procedure of determining jump height during the loaded countermovement jump exercise: Take-off velocity vs. flight time. J Strength Cond Res 32(7): 2025-2030, 2018-This study aimed to compare the reliability of jump height between the 2 standard procedures of analyzing force-time data (take-off velocity [TOV] and flight time [FT]) during the loaded countermovement (CMJ) exercise performed with a free-weight barbell and in a Smith machine. The jump height of 17 men (age: 22.2 ± 2.2 years, body mass: 75.2 ± 7.1 kg, and height: 177.0 ± 6.0 cm) was tested in 4 sessions (twice for each CMJ type) against external loads of 17, 30, 45, 60, and 75 kg. Jump height reliability was comparable between the TOV (coefficient of variation [CV]: 6.42 ± 2.41%) and FT (CV: 6.53 ± 2.17%) during the free-weight CMJ, but it was higher for the FT when the CMJ was performed in a Smith machine (CV: 11.34 ± 3.73% for TOV and 5.95 ± 1.12% for FT). Bland-Altman plots revealed trivial differences (≤0.27 cm) and no heteroscedasticity of the errors (R ≤ 0.09) for the jump height obtained by the TOV and FT procedures, whereas the random error between both procedures was higher for the CMJ performed in the Smith machine (2.02 cm) compared with the free-weight barbell (1.26 cm). Based on these results, we recommend the FT procedure to determine jump height during the loaded CMJ performed in a Smith machine, whereas the TOV and FT procedures provide similar reliability during the free-weight CMJ.