Background: Management of Chronic Inflammatory Rheumatic Diseases (CIRDs) provides the treatment with biological drugs, which are associated with an increase of Polyomavirus JC (JCV) reactivation. JCV is the aetiological agent of Progressive Multifocal Leukoencephalopathy (PML), a fatal demyelinating disease. Epidemiology of PML has been poorly characterized among patients with rheumatic diseases due to little population-based data existing. Methods: Blood and urine samples, belonged to 22 CIRDs subjects, were collected prior the first anti-TNF-α infusion (t0), 3 (t1) and 6 months later (t2), to evaluate a possible correlation between viral reactivation and treatment with anti-TNF-α and/or other immunosuppressive drugs. The JC viral load has been tested in each samples by real time quantitative PCR (QPCR). Results: At each time of follow up, the JC viruria was significantly higher than JC viremia (p = 0.022). Conclusions: Since PML epidemiology has been poorly characterized among patients with rheumatic diseases, this study contributes to enrich literature insight on JCV biology in this cluster of patients, considering that JCV involvement in development of adverse events in CIRDs is probably underestimated.