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Objective.Mycoplasmal infections are associated with several acute and chronic illnesses. Some mycoplasmas can enter a variety of tissues and cells, and cause system-wide or systemic signs and symptoms.Methods.Patients (14 female, 14 male) diagnosed with rheumatoid arthritis (RA) were investigated for mycoplasmal infections in their blood leucocytes using a forensic polymerase chain reaction (PCR) procedure. Amplification was performed with genus- and species-specific primers, and a specific radiolabelled internal probe was used for Southern hybridization with the PCR product. Patients were investigated for the presence of Mycoplasma spp., and positive cases were further tested for infections with the following species: M. fermentans, M. hominis, M. pneumoniae and M. penetrans.Results.The Mycoplasma spp. sequence, which is not entirely specific for mycoplasmas, was amplified from the peripheral blood of 15/28 patients (53.6%) and specific PCR products could not be detected in 13 patients (46.4%). Significant differences (P<0.001) were found between patients and positive healthy controls in the genus test (3/32) and in the specific tests (0/32). Moreover, the incidence of mycoplasmal infections was similar in female and male patients. using species-specific primers, we were able to detect infections with M. fermentans (8/28), M. pneumoniae (5/28), M. hominis (6/28) and M. penetrans (1/28) in RA patients. In 36% of the patients, we observed more than one Mycoplasma species in the blood leucocytes. All multiple infections occurred as combinations of M. fermentans with other species.Conclusions.The results suggest that a high percentage of RA patients have systemic mycoplasmal infections. Systemic mycoplasmal infections may be an important cofactor in the pathogenesis of RA, and their role needs to be explored further.Keywords:Rheumatoid arthritis, Chronic infections, Mycoplasma, Polymerase chain reaction.