Objectives To provide information of semen quality among normal young Japanese men and indicate the frequency of reduced semen quality.

Design Cross-sectional, coordinated studies of Japanese young men included from university areas. The men had to be 18–24 years, and both the man and his mother had to be born in Japan. Background information was obtained from questionnaires. Standardised and quality-controlled semen analyses were performed, reproductive hormones analysed centrally and results adjusted for confounding factors.

Setting Four study centres in Japan (Kawasaki, Osaka, Kanazawa and Nagasaki).

Participants 1559 men, median age 21.1 years, included during 1999–2003.

Outcome measures Semen volume, sperm concentration, total sperm count, sperm motility, sperm morphology and reproductive hormone levels.

Results Median sperm concentration was 59 (95% CI 52 to 68) million/ml, and 9% and 31.9% had less than 15 and 40 million/ml, respectively. Median percentage of morphologically normal spermatozoa was 9.6 (8.8 to 10.3)%. Small, but statistically significant, differences were detected for both semen and reproductive hormone variables between men from the four cities. Overall, the semen values were lower than those of a reference population of 792 fertile Japanese men.

Conclusions Assuming that the investigated men were representative for young Japanese men, a significant proportion of the population had suboptimal semen quality with reduced fertility potential, and as a group they had lower semen quality than fertile men. However, the definitive role—if any—of low semen quality for subfertility and low fertility rates remain to be investigated.