Adequate hydration is crucial in maintaining optimal physical and mental functioning and the need for a fast and reliable hydration status assessment in behavioral medicine research has become increasingly important. The goal of this study was to determine the reliability of bioelectrical impedance assessment (BIA) in assessing total body water (TBW), extracellular water (ECW) and intracellular water (ICW) and to assess whether individuals can be reliably classified as being hypohydrated or hyperhydrated using lower and upper quartiles, respectively. TBW, ECW and ICW were assessed via BIA (Bodystat, Isle of Man, UK) in 52 male and 48 female college students on 2 separate days within 1 week. Results revealed strong test-retest reliability for TBW (r=0.983), ECW (r=0.972) and ICW (r=0.988) (all P's<0.001). Following the initial and follow-up assessments, participants were then classified as being either hypohydrated or hyperhydrated based on the percentage of body weight accounted for by TBW. Test-retest reliability of hydration status within classifications was then assessed by gender. Test-retest reliability was found for TBW, ECW and ICW among hypohydrated (r=0.985, r=0.972 and r=0.99, respectively) and hyperhydrated (r=0.994, r=0.989 and r=0.994, respectively) males (all P's<0.001). Significant test-retest correlations were also found for females classified as being hypohydrated (r=0.97, r=0.956 and r=0.976, respectively) and hyperhydrated (r=0.973, r=0.976 and r=0.976, respectively) (all P's<0.001). These findings suggest that hydration status, as indexed by bioelectrical impedance technique, is reliable across time and is also reliable within individuals who are chronically hyperhydrated or hypohydrated.