Recent evidence indicates that Chlamydophila psittaci (Cp) may establish chronic infections, which may promote autoimmunity and/or B cell lymphoproliferation.The presence of a subclinical Cp infection was investigated in 293 patients with chronic inflammatory polyarthritis, including 175 patients with rheumatoid factor (RF)-positive and/or anti-CCP-positive rheumatoid arthritis (RA) and 118 with seronegative polyarthritis (46 RF-negative/anti-CCP-negative RA, 36 psoriatic arthritis and 36 undifferentiated spondyloarthritis). One hundred and eighty-five healthy controls were also investigated. The presence of Cp infection was assessed in peripheral blood mononuclear cells using several PCR protocols targeting different regions of the Cp genome (16S-23S spacer rRNA, OMP-A, and Gro-EL). The DNA of other Chlamydia species (C. Pneumoniae and C. Trachomatis) was also investigated. Amplicons were sequenced to confirm the specificity of PCR products.The presence of a subclinical chronic Cp infection was observed in a significantly higher percentage of patients with chronic polyarthritis (38/293; 13%) compared to healthy controls (1/185, 0.5%; OR=27.4, 95%CI:3.73-201.6, p<0.0001). Furthermore, the prevalence of Cp was higher in seronegative polyarthritis (23/118; 19.5%) than in seropositive RA patients (15/175; 7.4%; OR=2.58, 95%CI: 1.28-5.19, p=0.0078). The highest prevalence of Cp infection was found in RF/anti-CCP double-negative RA patients (13/46, 28.3%), followed by patients with psoriatic arthritis (6/36; 16.7%). No differences in age, sex, disease duration and undergoing therapies were noticed between Cp-positive and Cp-negative patients; nor between seropositive and seronegative patients.Cp may be an infectious trigger possibly involved in the pathogenesis of a fraction of inflammatory polyarthritis, particularly in seronegative patients.