Trichomonas tenax, cosmopolitan flagellate inhabiting human oral cavity, is the etiological agent of oral trichomonosis associated with gingival and periodontium deterioration. Purpose of this studies was to investigate the prevalence of infection with Trichomonas tenax identified by molecular techniques amplifying the region of ITS1-5.8S rRNA-ITS2 specific for T. tenax. The study included 498 persons: diabetic, renal transplant, rheumatoid arthritis patients and the control group. Prevalence of T. tenax in oral cavity was 10.2% in control group, 14.1% in diabetics, 12.0% in renal transplant patients and 14.0% in rheumatoid arthritis patients. Comparative assessment of results showed symptoms of gingiva and periodontium deteriorations, at varying intensity in patients with various systemic diseases; higher prevalence of the trichomonad infection was revealed in adults in all groups. Simultaneously, renal transplantation, diabetes, rheumatoid arthritis and related therapy do not affect T. tenax incidences and no increased risk of the infection has been observed in the patients; the permanent medication used due to main disease should be taken into consideration as likely inhibitory factor.