Wet preparation has limited sensitivity for diagnosis of Trichomonas vaginalis (TV) infection. An observational study of 337 women was conducted to evaluate a new polymerase chain reaction (PCR) test for TV. The sensitivities of wet preparation and TV culture were 52% (95% confidence interval [CI], 41-62) and 78% (95% CI, 69-86), respectively. TV PCR had a sensitivity of 84% (95% CI, 75-90) and a specificity of 94% (95% CI, 90-97). Metronidazole was provided to 67 (69%) of 97 women with TV because of TV on wet preparation, exposure to TV, or a diagnosis of bacterial vaginosis or pelvic inflammatory disease; however, if TV PCR had been used for diagnosis, 81 (84%) of 97 women with TV would have been treated (P=.02). TV is significantly undertreated using standard algorithms for metronidazole therapy. Given the association of trichomoniasis with perinatal morbidity and HIV transmission, women in high-risk groups may benefit from TV PCR.