Tight underwear (TU) may increase scrotal temperature and affect male reproductive health, but large-scale evidence remains limited. In this cross-sectional study, 26 836 men who self-reported wearing either loose underwear (LU; n = 21 221) or TU (n = 5615) were recruited. The primary objective was to assess the association between underwear type and semen quality, whereas the exploratory objective was to examine associations with erectile dysfunction and premature ejaculation using the International Index of Erectile Function-5 (IIEF-5) and Premature Ejaculation Diagnostic Tool (PEDT) questionnaires, respectively. Men in the TU group had higher prevalences of smoking, alcohol consumption, varicocele, and prostatitis. After adjustment, TU was associated with reduced semen volume (adjusted odds ratio [aOR] = 1.18, 95% confidence interval [CI]: 1.07–1.31, P = 0.002), decreased progressive motility (aOR = 1.16, 95% CI: 1.09–1.23, P < 0.001), decreased total motility (aOR = 1.13, 95% CI: 1.07–1.20, P < 0.001), reduced total motile sperm count (aOR = 1.12, 95% CI: 1.02–1.23, P = 0.01), decreased normal morphology rate (aOR = 1.48, 95% CI: 1.33–1.69, P < 0.001), and increased DNA fragmentation index (DFI; aOR = 2.08, 95% CI: 1.44–3.01, P < 0.001). In the exploratory analyses, erectile dysfunction (62.7% vs 57.8%) and premature ejaculation (30.8% vs 23.2%) were more prevalent in the TU group (erectile dysfunction, aOR = 1.18, 95% CI: 1.11–1.26, P < 0.001; and premature ejaculation, aOR = 1.40, 95% CI: 1.29–1.53, P < 0.001). In conclusion, TU was associated with poorer semen quality and a higher prevalence of erectile dysfunction and premature ejaculation. Choosing LU may represent a simple, modifiable approach to help protect semen quality and sexual function.

Keywords: erectile dysfunction; premature ejaculation; semen quality; sexual function; underwear