Little is known about the impact of behavioral changes after the 2003 heat wave on hydration status of elderly citizens in France. We used an administrative data file provided information about 23,022 inpatients aged > or =70 years admitted between 2000 and 2006, including vital status at discharge and Charlson comorbidity index and matched it with the result of five blood tests (sodium, potassium, glucose, urea nitrogen, creatinine) within the first 24 h after admission and with daily temperatures before admission. We then measured the prevalence of plasma tonicity (PT) <275 mOsm/l or >300 mOsm/l, blood urea nitrogen/creatinine ratio (BUNC) >100 and inhospital mortality. In 2000-2002, 2003, 2004-2006, prevalence (%) was, respectively 7.5, 8.0, 9.5 (P < 0.0001) for PT < 275 mMol/l, 8.4, 10.4, 7.2 (P < 0.0001) for PT > 300 mOsm/l, and 35.4, 30.7, 26.7 (P < 0.0001) for BUNC > 100. Inhospital mortality rate was 10.8, 10.8 and 9.0%, respectively (P < 0.0001). After adjustment for covariates, OR (95% CI) in 2004-2006 with reference to 2000-2002 was 1.26 (1.13-1.39) for PT < 275 mMol/l, 0.85 (0.76-0.94) for PT > 300 mOsm/l, and 0.65 (0.61-0.69) for BUNC > 100. Inhospital mortality risk associated with hydration disorders did not vary significantly over periods for PT < 275 mMol/l (HR 1.06 to 1.40) and PT > 300 mOsm/l (HR 1.76 to 1.96) but was lower for BUNC > 100 in 2003 (HR 1.27) than in 2000-2002 (HR 1.64) or 2004-2006 (HR 1.77) (P = 0.04). So, since the 2003 heat wave, significant shifts in prevalence of intracellular hydration disorders indicate behavioral changes with positive impact on hydration status.