Boron (B) levels were determined in the serum of 980 healthy inhabitants living in an urban area of Japan by means of inductively coupled plasma emission spectrometry (ICPES). The results showed a log-normal distribution of serum B for both sexes, although there are age-related differences. In male subjects, serum B increases rapidly up to 49 yr of age, reaching a plateau between ages 50 and 69 yr old, followed by a gradual increase up to 70 yr or older. Female subjects exhibit a gradual increase up to the age of 70 yr old. The reference value for male and female subjects was 79.8 micrograms/L and 67.9 micrograms/L, and the reference interval was 33.3-191.2 micrograms/L and 29.5-154.9 micrograms/L, respectively. The obtained reference value and interval of the nonexposed group may be useful for health screening for B exposure, either for people living in regions with high levels of B in the environment, or for workers who are exposed to this element.