The conventional Papanicolaou smear (CPS) is not considered accurate for the diagnosis of Trichomonas vaginalis (T. vaginalis), and women noted to carry the organism on CPS are recommended to undergo confirmatory testing. Liquid-based preparations have been shown to facilitate the diagnosis of squamous lesions and may also facilitate the diagnosis of T. vaginalis. We used polymerase chain reaction (PCR) to investigate the accuracy of the diagnosis of T. vaginalis by the liquid-based Pap test (LBP). LBP with the diagnosis of T. vaginalis from a 12-mo period were identified. Residual samples from these cases were subjected to PCR for T. vaginalis as were the residual samples of a control group of 195 LBP (including 103 inflammatory LBP and 69 cases of atypical squamous cells) in which T. vaginalis was not diagnosed cytologically. PCR confirmed the presence of T. vaginalis in 50 of 51 (98%) LBP and identified 2 additional cases. Morphologic identification of T. vaginalis on LBP is highly accurate and should not require confirmatory testing.