Mononuclear cells play a cardinal role in the pathogenesis of systemic lupus erythematosus (SLE). A high urine cytology score has been reported to be associated with lupus nephritis in relapse. The objective of this study was to examine the urinary mononuclear cell population of patients with lupus nephritis, and explore its correlation with lupus disease activity. We studied 12 patients with active lupus nephritis, 17 patients with lupus nephritis in remission, 12 SLE patients with no history of renal disease and 13 healthy subjects. Clinical disease activity was quantified by the SLE Disease Activity Index (SLEDAI). Mononuclear cell species in the urinary sediment were examined by immunocytochemistry. Patients with active lupus nephritis had significantly more mononuclear cells in the urinary sediment. The number of + cell was significantly elevated in the active lupus nephritis than the others ( P < 0.001), while there was no significant difference in the number of + and + cell among patient groups. The total urinary mononuclear cell correlated significantly with the overall SLEDAI score ( r = 0.58, P < 0.001) as well as the renal score ( r = 0.57, P < 0.001). The number of urinary +, but not + or +, cell significantly correlated with the overall SLEDAI score ( r = 0.46, P = 0.003) as well as the renal score ( r = 0.40, p < 0.011). In nine patients with renal biopsy, the histological activity index correlated with the total urinary mononuclear cell ( r = 0.75, P = 0.02), + ( r = 0.69, P = 0.04) and + cell ( r = 0.69, P = 0.04). We conclude that urinary mononuclear cell was markedly elevated in patients with active lupus, and the urinary mononuclear cell count correlated significantly with the SLEDAI score and histological activity. + and + cells are the major component of urinary mononuclear cell in SLE patients and their number correlates with lupus disease activity.