Silent lupus nephritis (SLN) was investigated in 42 renal asymptomatic patients and compared with 49 untreated patients with overt lupus nephropathy (OLN). Urinary sediment, quantitative proteinuria, creatinine clearance, antinuclear antibodies (ANA), complement, circulating immune complexes (CIC) and renal biopsies were evaluated in all of the patients. Forty-one out of the 42 (97.6%) patients had SLN according to histopathological findings. Results showed that the mean age, female/male ratio and the clinical activity index (SLEDAI) were similar in both groups (P > 0.05). The prevalence of ANA, anti-ds DNA, anti-ENA autoantibodies and C4 serum levels showed no statistical differences between the two groups (P > 0.05). Conversely, in the OLN group, elevated CIC and diminished CH50 and C3 serum levels were significantly different (P < 0.01). WHO class II was the predominant renal lesion in the group with SLN (P < 0.0001), whereas class IV was in the OLN patients (P < 0.0001). We conclude that, in our series, SLN was highly prevalent in renal asymptomatic patients with otherwise systemic lupus erythematosus. Furthermore, abnormal levels of CIC, CH50 and C3 associated with WHO class II suggest a moderate but ongoing activation of immune-mediated renal injury mechanisms.