The aim of the present study was to investigate the trend for obesity and the relationship between Trp64Arg mutation in the β 3 -adrenergic receptor gene and obesity-related phenotypes in Japanese workers. A total of 658 workers, 483 women and 175 men, responded to this survey. Genomic DNA was prepared from leukocytes, and DNA amplification by polymerase chain reaction was performed using 100 ng genomic DNA. Amplified fragments were digested with BstNI and analyzed by 3% agarose gel electrophoresis. The body mass index (BMI) increased from 21.1 +/- 2.6 to 22.0 +/- 3.0 kg/m 2 for women and from 20.9 +/- 2.1 to 22.1 +/- 2.6 kg/m 2 for men from 1982 to 1997. Body weight increased 63.3% for women and 67.4% for men over 15 years. The frequency of the Trp64Arg allele was 19.9%, and the frequency of heterozygote and homozygote carriers of the mutation was 32.9% and 3.4%, respectively. Changes in the BMI and body weight over 15 years from 1982 to 1997 did not differ significantly between subjects with or without the mutant allele in either women or men. The BMI was not significantly higher in subjects who were homozygous or heterozygous for the Trp64Arg mutation versus subjects who were homozygous for the normal allele. This mutation of the β 3 -adrenergic receptor gene is not likely a major determinant of moderate obesity in the Japanese population.