The behavior of fluoride ion level in the urine and in the serum was measured as an index for health care of workers exposed to hydrofluoric acid, the effect of fluorine-containing foodstuff intake by the workers being considered at the same time. For this study, 250 hydrofluoric acid workers and as a control group 1,600 non-hydrofluoric-acid workers were examined. All workers, ranging in age from 15 to 59 years old, came from the same electronics factory. Fluorides in the biological fluids were measured by using the fluoride ion-specific electrode method and the electrode potential recording method concurrently. The fluoride ion levels (geometric mean) in the urine and in the serum of the non-hydrofluoric-acid workers were determined to be 0.59 ppm and 12 ppb, respectively. Among the influences of fluorine-containing foodstuff on fluoride content in the biological fluids, the effect of black tea and/or green tea intake was particularly remarkable. In the case of the non-hydrofluoric-acid workers, the concentration increased to about double of the control value. Similarly in a diet test on volunteers, the concentration increased about six times. As for the response to tea intake with the lapse of time, the concentration in the urine specimen (spot urine, S.G. corrected: 1.024) reached the maximum value three hours after the intake. In the case of the serum specimen, the maximum value was recorded one hour after the intake, and the influence of the intake continued for several hours. These results suggest that, in the medical examination of hydrofluroic acid workers, restriction of intake of tea and in some cases even a fasting program would have to be introduced as necessary pre-conditions. Upon observation of fluorine concentration in the urine with the lapse of time of hydrofluoric acid workers restricting tea intake, it was confirmed that the trend of the day-to-day variation was small. But that of the in-a-day variation, namely, an increase in the concentration toward the end of daily working hours was obvious. By introduction of these well-regulated methods, it has become possible to properly evaluate even the influence of hydrofluoric acid on the workers irregularly exposed to a hydrofluoric acid environment having a lower concentration than that of the threshold limit value (3 ppm). However, in view of ease of collection and measurement of specimens, speed, and degree of response to the exposure, it can be concluded that the measurement of fluorine content in the urine is more advantageous as an index for the health care than that in the serum. For this reason it is desirable that the former method should be given priority from the view point of prevailing industrial hygiene laws and regulations.