The diagnosis of Trichomonas vaginalis (TV) infection in women may be made by observing motile trichomonads in wet-mount preparations of vaginal discharge. The duration for which these organisms can be identified in such samples is unknown. We sought to assess this by performing wet-mount microscopy on samples from female patients immediately, and then in positive samples, every 10 minutes until motile trichomonads could no longer be identified. Of the 65 wet-mount specimens positive for TV, a cumulative total of 13 (20%) samples had become negative by 10 minutes, 23 (35%) samples by 30 minutes and 51 samples (78%) by two hours, with the remainder exceeding two hours. We conclude that one-fifth of wet-mount vaginal preparations initially positive for motile TV become negative within 10 minutes of the initial, immediate reading. In order to maximize the sensitivity of this widely used test it is recommended that all specimens be examined immediately after they are taken.