Photo: flickr/dave77459The answer: more than 20(!), according to this study of over 3,000 men out of Montreal, Canada -- but only if they're female. (Apparently, having more than 20 male sexual partners is associated with an increased risk of prostate cancer.) Although this is just a correlation, it's certainly interesting. And if it's not the number of sexual partners that reduces prostate cancer risk, but something else these ladies men have in common, we definitely want to know what that is! But, meanwhile, do you know your number? Sexual partners, sexually transmitted infections, and prostate cancer risk "Background: The etiology of prostate cancer (PCa) is poorly understood. Sexual activity and sexually transmitted infections (STIs) are among factors under scrutiny, with controversial findings to date. Methods: We examined the association between the number and gender of sexual partners, STIs and PCa risk in the context of PROtEuS, a population-based case–control study set amongst the mainly French-speaking population in Montreal, Canada. The study included 1590 histologically-confirmed PCa cases diagnosed in a Montreal French hospital between 2005 and 2009, and 1618 population controls ascertained from the French electoral list, Montreal residents, frequency-matched to cases by age. In-person interviews elicited information on sociodemographic, lifestyle and environmental factors. Unconditional logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs) between sexually related factors and PCa risk, adjusting for age, ancestry, family history of PCa, and PCa screening history. Results: Subjects with more than 20 sexual partners in their lifetime had a decreased risk of PCa (OR 0.78, 95% CI 0.61–1.00) as did subjects who specifically had more than 20 female sexual partners (OR 0.72, 95% CI 0.56–0.94). By contrast, having had several male sexual partners appeared to confer some excess in risk of PCa. No association emerged for history of STIs and PCa but STIs prevalence was low. Conclusion: Our findings are in support of a role for the number of sexual partners in PCa development. The gender of sexual partners should be taken into account in future studies investigating this association." Related content: NCBI ROFL: Does bestiality cause penile cancer?NCBI ROFL: Does pizza cause cancer?NCBI ROFL: 8% of Swedish men are peeping Toms.