Polyomavirus BK (BKV) reactivation can occur in immunodeficient patients. Few studies on BKV infection in patients with systemic lupus erytematosus (SLE) nephritis are available. Aim of this study was to analyse the prevalence of BKV infection by quantifying viral load and to investigate the association with clinical and histological parameters indicating duration, type and activity of SLE.BKV-DNA was evaluated by polymerase chain reaction in serum (sBKV) and urine (uBKV) specimens from 40 patients with SLE nephritis and 29 healthy controls. Renal function, urinary activity, clinical index of SLE activity [SLE Disease Activity Index (SLEDAI) score], CD4+/CD8+ ratio, histological classes and duration of SLE nephritis were compared according to the BKV-DNA-positivity.sBKV was present in 15% of SLE patients and in 13.8% of controls; uBKV in 32% of SLE patients and in 17.2% of controls. There was no significant difference in terms of kidney function, urinary activity, SLEDAI score, presence of anti-dsDNA antibodies, CD4+/CD8+ ratio and BKV viremia and/viruria, as well as there was no significant correlation between SLEDAI score, anti-dsDNA antibodies titers and median viral load. Duration of nephropathy tended to be shorter in patients with BKV viremia and/or viruria; proteinuria/creatininuria ratio tended to be higher in patients with positive sBKV and uBKV. BKV-DNA-positivity tended to be more frequent in patients treated with an immunosuppressive agent versus those on steroid treatment. Reactivation of BKV infection can occur in patients with SLE, although prevalence data do not significantly differ from those obtained in the control group. The trend toward an association between BKV infection and degree of proteinuria and less duration of SLE nephritis could indicate a major susceptibility to develop BKV infection in more active phases of the disease. The role of BKV reactivation in terms of clinical parameters and histological pattern, as well as the role of therapeutic protocols in the onset of BKV reactivation and, conversely, the therapeutic implication of BKV reactivation in SLE patients remain to be defined and should be addressed in further studies on a larger number of patients.