BACKGROUND: The interdialytic weight gain (IDWG) and protein catabolic rate (PCR), expressed per 24 hours (daily), are usually assumed to be stable over the week in chronic hemodialysis (HD) patients. METHODS: We studied different HD patient groups at different time points (study 1 in 1993: n = 48, study 2 in 1999: n = 16, study 3 in 2000: n = 112). Daily IDWG (calculated from post- and pre-dialysis body weight and interdialytic interval) and nPCR (calculated from kinetic modeling) were compared over short (2 days) and long (3 days) intervals. RESULTS: In all groups of HD patients studied, both daily IDWG and nPCR were significantly (p < 0.05) lower (IDWG by 7-15%, nPCR by 5-6%) over long than short interdialytic intervals. This difference was observed whether or not blood sampling was planned after the long interval. This suggests selfrestriction of dietary intake over the long interdialytic interval. CONCLUSIONS: Daily IDWG and nPCR are lower over the long interdialytic interval. Studies should thus define or even standardize the period of time over which IDWG and nPCR are calculated