During 1978 a survey was carried out on 34 036 people in Nagano Prefecture, Japan, in an epidemiological study of hyperuricaemia; 15 713 males and 18 324 females were examined by means of our health examination car. The subjects of the survey were all volunteers. The results have shown that the serum uric acid (SUA) levels were related to sex and age. The male group had a mean SUA level of 5.82 mg/dl (range 0.9 to 11.8 mg/dl, standard deviation 1.14 mg/dl) (0.35, range 0.05-0.7, SD 0.07 mmol/l), and the female group had a mean of 4.33 mg/dl (range 0.5 to 9.7 mg/dl, standard deviation 0.91 mg/dl) (0.26, range 0.03-0.58, SD 0.05 mmol/l). The distribution curve for SUA was almost normal in both sexes. Lower levels of SUA were found in males from the 2nd to the 6th decade, but they increased again after the 7th decade. In females they gradually decreased and were at the lowest in the 4th and 5th decades, and then increased again. The values were always lower in females than males with the difference ranging from 1.84 to 0.81 mg/dl (0.11-0.05 mmol/l).