Background. Trichomonas vaginalis causes a common sexually transmitted infection (STI) in women, yet trichomoniasis in male sexual partners is not well recognized. Nucleic acid amplification tests can increase detection of T. vaginalis in men compared with culture. Methods. We conducted a prospective, multicenter study to evaluate T. vaginalis infection among male partners of women with trichomoniasis and factors associated with infection by recruiting patients from 3 public clinics in the United States. Male partners were tested for concordant T. vaginalis infection, defined as a positive urethral culture, urine culture, or urine polymerase chain reaction (PCR) result. A subset of men also provided a semen sample for T. vaginalis culture and PCR. Factors associated with concordant infection were determined from bivariable and multivariable analyses. Results. We enrolled 540 women with trichomoniasis (diagnosed using wet mount microscopy and/or culture) and 261 (48.4%) of their male partners. T. vaginalis infection was detected in 177 (71.7%) of 256 male partners (95% confidence interval [CI], 66.0%-77.3%), of whom 136 (77.3%) were asymptomatic. A vaginal pH of >4.5 in a woman was independently associated with infection in the male partner (adjusted odds ratio, 2.5; 95% CI, 1.0-6.3). Younger male age (20-29 and 30-39 years) was also found to be an independent risk factor for concordant trichomoniasis. Conclusions. The majority of male partners of women with trichomoniasis were infected; however, few factors predicted infection. T. vaginalis causes a highly prevalent STI, necessitating vastly improved partner management, application of sensitive nucleic-acid based testing, and better clinical recognition. © 2006 by the Infectious Diseases Society of America. All rights reserved.