OBJECTIVE Trichomonas vaginalis is the causative agent of trichomoniasis, one of the most common sexually transmitted diseases worldwide. In recent years we have described the symbiotic relationship between T vaginalis and Mycoplasma hominis. How this biological association might affect the pathogenicity of one or both the microorganisms is still unknown. Since local inflammation is thought to play a central role in T vaginalis infection, we investigated the in vitro response of human macrophages to naturally mycoplasma-free T vaginalis, as compared to a mycoplasma-infected trichomonad isolate. METHODS THP-1 cells were stimulated with two isogenic T vaginalis isolates, one naturally mycoplasma-free and one stably associated with M hominis, and secreted cytokines measured by ELISA. Nuclear factor κB (NFκB) involvement in THP-1 response to T vaginalis and M hominis was evaluated by means of a reporter system based on detection of alkaline phosphatase activity. RESULTS We found that the presence of M hominis upregulates the expression of a panel of proinflammatory cytokines in a synergistic fashion. We also found that the upregulation of the proinflammatory response by THP-1 cells involves the transcription factor NFκB. CONCLUSIONS These findings suggest that the presence of M hominis in T vaginalis isolates might play a key role in inflammation during trichomoniasis, thus affecting the severity of the disease. The synergistic upregulation of the macrophage proinflammatory response might also affect some important clinical conditions associated with T vaginalis infection, such as the increased risk of acquiring cervical cancer or HIV, which are thought to be affected by the inflammatory milieu during trichomoniasis.