Objective: To study the effect of body mass index (BMI) on semen quality in a cohort of male partners in subfertile couples. Design: Prospective cohort study. Setting: A fertility center based in an academic hospital. Patient(s): Between January 2000 and January 2007, 1466 men visiting the Centre for Reproductive Medicine as part of a subfertile couple. Intervention(s): None. Main Outcome Measure(s): Semen volume (in mL), semen concentration (in millions per mL), percentage of motile spermatozoa, percentage of spermatozoa with normal forms, total sperm count (in millions), and total motile sperm count (in millions). Result(s): After exclusion of men without data on BMI, the data of 1401 men could be analyzed. The group of men with a BMI lower than 20 kg/m 2, with a BMI between 25 and 30 kg/m2, and with a BMI >30 kg/m2 had a lower semen volume compared with the group with a BMI between 20 and 25 kg/m2. Other semen parameters were not statistically significantly different. Multivariable analysis (generalized linear model), correcting for confounders, showed no statistically significant association between BMI and semen parameters, including semen volume. Conclusion(s): Semen quality was not statistically significantly affected by BMI in a cohort of male partners in subfertile couples.