Prevalence of diabetes mellitus in Japan was estimated from fructosamine and hemoglobin A1c measurement data obtained from the National Survey of Circulatory Disorders 1990, using a previously reported method which used the results of a screening test for estimating true prevalence. The prevalence for males was higher than that for females. The prevalences were less than 10% for those 49 years or younger and approximately 20% for those 50 years or older. The estimated values derived from fructosamine or hemoglobin A1c did not vary considerably.