The objective of this study was to determine the quality of semen in Japanese men born in the 1970's. When samples were compared among generations (1970-72, 1973-75 and 1976-78), significant differences were not seen in the volumes and the abnormality rates. But the sperm concentration of the 1970-72 generation (114.8 ± 52.5 × 106 sperms/mL n=3) was higher than these of other generations (48.8 ± 44.1 × 106 and 58.0 ± 27.0 × 106 sperms/mL, respectively 1973-75 n=17 and 1976-78 n=28). There was no correlation between semen quality (sperm concentration and total sperm number) and the date of birth of the subjects. However their parents' date of birth had an influence on semen quality. Environmental chemicals began to increase around the time the subjects' parents were born, and it might be possible that parents' reproductive ability was connected with their children's semen quality.