Aim of the study was to analyze temporal trends in prevalence of hyponatremia over four decades in different settings. A systematic review of the literature from 1966 to 2009 yielded prevalences of hyponatremia, with standard errors (SE) and pooled estimated means (PEM), calculated by year and setting (geriatric, ICU, other hospital wards, psychiatric hospitals, nursing homes, outpatients). 53 studies were included. Prevalence of hyponatremia was stable from 1976 to 2006, and higher on geriatric wards accept for ICU: e.g. PEM prevalence of mild hyponatremia (serum sodium <135. mM) was 22.2% (95%CI 20.2-24.3) on geriatric wards, 6.0% (95%CI 5.9-6.1) on other hospital wards and 17.2% (SE 7.0) in one ICU-study; for severe hyponatremia (serum sodium < 125. mM) these figures were 4.5% (95%CI 3.0-6.1), 0.8% (95%CI 0.7-0.8) and 10.3% (SE 5.6). In nursing homes PEM prevalence of mild hyponatremia was 18.8% (95%CI 15.6-22.2). The higher prevalence on geriatric wards could partly be explained by age-related changes in the regulation of serum sodium. Other underlying factors can be the presence of multiple diagnoses and the use of polypharmacy.