In this study we present the results of in vivo measurements of the mercury concentration in organs of occupationally exposed persons by the use of X-ray fluorescence analysis (XRF). The mercury concentration in the right kidney was measured in 20 occupationally exposed workers and 12 referents. The detection limit for the individual persons varied with the kidney depth (mean 26 μg/g, range 12-45 μg/g), was exceeded in nine of the exposed workers but in none of the referents. The mean kidney mercury concentration (including estimated concentration values below the detection limit) was 24 μg/g in the group of exposed workers (group MDC 5 μg/g) and for the group of referents no detectable concentration (mean 1 μg/g) was found (group MDC 6 μg/g). The mean urinary mercury excretions for the two groups were 34 and 1.7 μg per g creatinine. X-ray fluorescence measurements made on liver (n = 10) and thyroid (n = 8), in some of the exposed workers, revealed no measurable mercury concentrations.