Colonization of human lungs by various Trichomonas species is a frequent occurrence, but is unknown to most physicians. At this site of infection, the parasite develops into an amoeboid form that renders it unrecognizable. For this reason it has been overlooked until recently. Morphological identification is not feasible under these conditions and molecular tools provide the only means of identification. The species involved are not restricted to Trichomonas tenax, a saprophyte of the mouth that is usually cited in the rare cases of pleuropulmonary trichomoniasis reported in the literature. The recent discovery of species previously unknown in humans raises further questions, including the zoonotic potential of these microorganisms and the existence of species of animal origin that have adapted to humans. Anaerobiosis in poorly ventilated alveolar lumen, rather than immunodepression, seems to be the factor that promotes proliferation of this parasite. The diagnosis of trichomoniasis and its treatment by specific drugs will make it possible to evaluate the pathogenicity of these parasites.