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Objective: To evaluate the influence of body mass index (BMI) on semen characteristics., Design: Cohort study., Setting: Single private andrology laboratory., Patient(s): All patients (n=10,665) consulting for a semen analysis from October 9, 2010, to October 8, 2011. When analyses were repeated on the same patient, only the first was included., Intervention(s): Recording of self-reported weight and height and of semen analysis., Main Outcome Measure(s): All parameters of standard semen analysis: pH, volume, sperm concentration per mL, total sperm count per ejaculate, motility (%) within 1 hour after ejaculation (overall and progressive), viability (%), and normal sperm morphology (%). Parametric and nonparametric statistical methods were applied, and results are given either with mean±SD, or 10th, 50th, and 90th percentiles., Result(s): Semen volume decreased from 3.3±1.6 to 2.7±1.6 mL when BMI increased from normal (20-25 kg/m2) to extreme obesity (>40 kg/m2). The same was true for semen concentration (56.4±54.9 to 39.4±51.0 million/mL), total sperm count (171±170 to 92±95 million), and progressive motility (36.9±16.8% to 34.7±17.1%). The percentage of cases with azoospermia and cryptozoospermia increased from 1.9% to 9.1% and from 4.7% to 15.2%, respectively. The other semen characteristics were not affected. Multivariate models including age and abstinence duration confirmed these results., Conclusion(s): In this study, on a large patient sample size, increased BMI was associated with decreased semen quality, affecting volume, concentration, and motility. The percentage of normal forms was not decreased., (Copyright © 2014 American Society for Reproductive Medicine. Published by Elsevier Inc. All rights reserved.)