OBJECTIVE: To investigate the association between childhood body mass index (BMI), birth weight, and adulthood BMI, and adult semen quality and level of reproductive hormones. DESIGN: Follow-up study. SETTING: From a pregnancy cohort established in 1984-1987. PATIENT(S): 347 out of 5,109 sons were selected for a study conducted 2005 to 2006. INTERVENTION(S): Semen and blood samples were related to information on BMI in boys (5-8 years), birth weight, and adult BMI. MAIN OUTCOME MEASURE(S): Semen characteristics and reproductive hormones. RESULT(S): Neither childhood BMI, birth weight, nor adulthood BMI were significantly associated with semen quality. Men with the 33% highest childhood BMI had 15% lower sex hormone binding globulin, 8% lower testosterone, and 16% lower FSH than men with the 33% lowest childhood BMI. Men with high adulthood BMI had 14% lower testosterone, 9% lower inhibin B, 31% lower sex hormone binding globulin, and 20% higher estradiol than men with low adulthood BMI. CONCLUSION(S): The results do not indicate an effect of childhood BMI, birth weight, or adult BMI on semen quality, but the exposure contrast in our study was limited. The hormonal status was affected by adult BMI.