To assess the significance of the serum dehydroepiandrosterone sulfate (DHEAS) concentration as a parameter of the individual hormonal milieu, two different groups of subjects were studied: DHEAS levels were determined 3 times at 6-month intervals in 47 elderly hospitalized women, aged 71-94 yr (group 1), and 6 times over 2 consecutive weeks in 10 healthy male volunteers, aged 24-30 yr (group 2). For reference, serum cortisol (F) levels were determined concomitantly. In each group and on each sampling occasion, the subjects were ranked according to their DHEAS or F values. The stability over time of the ranking was much higher for DHEAS than for F; estimated concordance coefficients were 92% (group 1) and 88% (group 2) for DHEAS vs. 51% (group 1) and 49% (group 2) for F. We conclude that due to a comparatively low within- to between-subject variability ratio, DHEAS is a highly specific individual marker.