Trichomonas vaginalis is a common sexually transmitted protozoan parasite. Although often considered simply a nuisance infection, T. vaginalis has been implicated in premature rupture of placental membranes and increases in the risk of acquiring human immunodeficiency virus. Metronidazole, a 5-nitroimidazole, is currently the drug of choice to treat T. vaginalis infection. Because some patients have severe reactions to metronidazole and others are infected with metronidazole-resistant T. vaginalis, we were prompted to investigate alternative therapies. Tinidazole, another 5-nitroimidazole used in other countries to treat T. vaginalis infections, and furazolidone, a nitrofuran presently used to treat giardiasis and infections with some anaerobic enteric bacteria, were investigated for effectiveness against 9 metronidazole-susceptible and 12 metronidazole-resistant T. vaginalis patient isolates. The in vitro aerobic and anaerobic minimum lethal concentrations (MLC) and the time for drug efficacy were determined. Tinidazole killed the metronidazole-susceptible isolates at a low MLC but was effective against only 4 of the 12 metronidazole-resistant isolates. In contrast, furazolidone was effective at a low MLC for all isolates. When tinidazole was effective, it required > 6 h to kill trichomonads. However, furazolidone killed both metronidazole-susceptible and resistant trichomonads within 2 to 3 h of exposure. These data suggest that furazolidone may be a good candidate for treating metronidazole-resistant trichomoniasis and that further investigation of this drug is warranted.