Body mass index (BMI) has been demonstrated to affect female fertility; however, little information is available on the impact of BMI on male fertility or semen parameters. Therefore, the study objective was to determine the relationship between BMI and semen parameters, including sperm chromatin integrity. We analyzed data on semen samples from 520 men who were grouped based upon calculated BMI values (normal, 20-24 kg/m(2); overweight, 25-30 kg/m(2); obese, >30 kg/m(2)). The data collected included patient height and weight, semen volume, sperm concentration, percent sperm motility, percent sperm morphology (normal forms), and sperm chromatin integrity (DNA fragmentation index [DFI]). Data were analyzed by regression analysis and analysis of variance (ANOVA) with Tukey's test for multiple pairwise comparisons. The overall BMI mean (+/-SEM) was 27.5 (+/-0.49) kg/m(2). Linear regression revealed a significant (P < .05) and negative relationship between BMI and the total number of normal-motile sperm cells. ANOVA revealed a significant difference (P < .05) in the total number of normal-motile sperm cells among the different BMI groups. The number of normal-motile sperm cells per BMI group was as follows: normal, 18.6 x 10(6); overweight, 3.6 x 10(6); and obese, (0.7) x 10(6). All multiple pairwise comparisons were found to be significantly (P < .05) different. The overall DFI mean (+/-SEM) was 24.7 (+/-2.57). Linear regression revealed a significant (P < .05) and positive relation between BMI and DFI. Men presenting with a BMI greater than 25 kg/m(2) have fewer chromatin-intact normal-motile sperm cells per ejaculate. Therefore, to ensure maximum fertility potential, patients may be advised to reduce body weight.