The finding of trichomonads in bronchoalveolar lavage fluid (BALF) samples from an acute respiratory distress syndrome (ARDS) patient, never previously reported, incited us to search for these parasites retrospectively in the BALF of patients with ARDS or related pathologies. Eighty-four consecutive BALF samples have been reviewed. Results were compared with data from clinical files of patients included in this study. Detection and identification of trichomonads were based on cytologic characteristics. Subsequently, immunocytochemistry and in situ hybridization were performed in the last case of the series. Our results were as follows: (1) Trichomonads were detected in 25/84 BALFs (20/77 patients). Among the patients testing positive, 17 suffered from ARDS, about 30% of the ARDS patients included in the study. (2) Trichomonads were detected more frequently at a late ARDS stage. (3) No correlation was found between trichomonad detection and other data. (4) Within the group of trichomonad-infected ARDS patients, density of infection correlated with higher mortality. The late detection of these microorganisms in the course of ARDS suggested that trichomonad development is a secondary event. As BALFs obtained early in the course of ARDS were also included in the study, trichomonad incidence could be underestimated. The significance of trichomonad lung colonization in ARDS and its potential pathogenic role are unclear. Nevertheless, the question of an active role of trichomonads in the extension of alveolar lesions or in the limitation of recovery is clearly raised.