Preworkshift and postworkshift urinary fluoride concentrations were measured in 142 hydrofluoric acid (HF) workers and 82 unexposed workers aged 18 to 59. Postshift urinary fluoride concentration in HF workers was significantly higher than that in preshift or control workers. A linear relationship was observed between the mean values in urinary fluoride concentration and the HF concentration in the air. The mean urinary fluoride concentration of 4 ppm and its lower fiducial limit (95%, P = 0.05) of 2 ppm (specific gravity 1.024) were estimated corresponding to the atmospheric HF concentration of 3 ppm, which is the maximal allowable concentration recommended by the Japanese Association of Industrial Health and also the threshold limit value suggested by the American Conference of Governmental Industrial Hygienists. The results suggest that exposure to HF can be monitored by determining the urinary fluoride concentration.