This study examined vertical jump performance using a force platform and weighted vest to determine why hypohydration (approximately 4% body mass) does not improve jump height. Measures of functional performance from a force platform were determined for 15 healthy and active males when euhydrated (EUH), hypohydrated (HYP) and hypohydrated while wearing a weighted vest (HYP(v)) adjusted to precisely match water mass losses. HYP produced a significant loss of body mass [-3.2 +/- 0.5 kg (-3.8 +/- 0.6%); P < 0.05], but body mass in HYP(v) was not different from EUH. There were no differences in absolute or relative peak force or power among trials. Jump height was not different between EUH (0.380 +/- 0.048 m) and HYP (0.384 +/- 0.050 m), but was 4% lower (P < 0.05) in HYP(v) (0.365 +/- 0.52 m) than EUH due to a lower jump velocity between HYP(v) and EUH only (P < 0.05). However, vertical ground reaction impulse (VGRI) was reduced in both HYP and HYP(v) (2-3%) compared with EUH (P < 0.05). In conclusion, this study demonstrates the failure to improve jump height when HYP can be explained by offsetting reductions in both VGRI and body mass.