SUMMARY Establishing reference norms for semen parameters in fertile men is important for accurate assessment, counselling and treatment of men with male factor infertility. Identifying temporal or geographic variability in semen quality also requires accurate measurement of semen parameters in well-characterized, defined populations of men. The Study for Future Families (SFF) recruited men who were partners of pregnant women attending prenatal clinics in Los Angeles CA, Minneapolis MN, Columbia MO, New York City NY and Iowa City IA. Semen samples were collected on site from 763 men (73% White, 15% Hispanic/Latino, 7% Black and 5% Asian or other ethnic group) using strict quality control and well-defined protocols. Semen volume (by weight), sperm concentration (hemacytometer) and sperm motility were measured at each centre. Sperm morphology (both WHO, 1999 strict and WHO, 1987) was determined at a central laboratory. Mean abstinence was 3.2 days. Mean (median; 5th–95th percentile) values were: semen volume, 3.9 (3.7; 1.5–6.8) mL; sperm concentration, 60 (67; 12–192) 9 10 6 /mL; total sperm count 209 (240; 32–763) 9 10 6 ; % motile, 51 (52; 28–67) %; and total motile sperm count, 104 (128; 14–395) 9 10 6 respectively. Values for sperm morphology were 11 (10; 3–20) % and 57 (59; 38–72) % normal forms for WHO (1999) (strict) and WHO (1987) criteria respectively. Black men had significantly lower semen volume, sperm concentration and total motile sperm counts than White and Hispanic/ Latino men. Semen parameters were marginally higher in men who achieved pregnancy more quickly but differences were small and not statistically significant. The SFF provides robust estimates of semen parameters in fertile men living in five different geographic locations in the US. Fertile men display wide variation in all of the semen parameters traditionally used to assess fertility potential.