Introduction Sexual partner concurrency has been shown to be a risk factor for a number of sexually transmitted infections but it is unknown if it is a risk factor for Trichomonas vaginalis (TV).

Study design We used mixed effects logistic regression to assess the association between partner concurrency and incident TV in the Longitudinal Study of Vaginal Flora, a cohort study of 3620 women followed quarterly for five visits.

Results TV was more common in those reporting definite/possible/unknown PC (15.6%/15.0%/18.3%) than those reporting no PC (5.2% - P<0.001 for all three comparisons). After controlling for a range of confounders, incident TV remained associated with reporting that one’s partner definitely (adjusted Odds ratio [aOR] 5.4; 95% Confidence Interval [CI], 3.7–8.0) and possibly (aOR 3.4; 95% CI, 2.2–5.1) engaged in partner concurrency in the preceding time period.

Conclusion Partner concurrency was associated with incident TV infection. We hypothesise that this association may be partly explained by concurrent partnering bypassing a ‘rapid-clearance-in-males-bottleneck.’