BACKGROUND In patients with lupus nephritis, mononuclear inflammatory cells infiltrate the renal interstitium and glomeruli, and the degree of leukocyte infiltration correlates with the severity of the renal dysfunction. The precise mediator signaling inflammatory cells to migrate into the kidney is not known. Recent findings that monocyte chemoattractant protein-1 (MCP-1), a chemotactic cytokine with a high degree of specificity for lymphocytes and monocytes, is overexpressed in glomeruli from rats with immunecomplex glomerulonephritis prompted us to explore the possibility that MCP-1 could be implicated in the renal inflammatory response of lupus erythematosus. METHODS Serum and urine levels of MCP-1 were evaluated in 10 patients with active lupus nephritis. Patients were studied at admission, before therapy, and at various time points after the first administration of high dose methylprednisolone. There was an additional observation of the four patients who underwent remission of clinical signs of the disease after chronic steroid treatment. Three additional groups, one (n = 9) of patients with inactive lupus nephritis, one (n = 9) of patients with nonlupus glomerulonephritis and high degree proteinuria, and one (n = 10) of healthy subjects, were also studied. RESULTS In patients with active lupus nephritis, urinary MCP-1 was significantly higher than in lupus patients studied in the inactive phase of the disease or in healthy volunteers. High doses of i.v. methylprednisolone significantly lowered urinary MCP-1 in patients with active lupus nephritis. In patients undergoing remission of lupus nephritis after chronic steroid treatment, urinary MCP-1 did not correlate with either serum MCP-1 levels or proteinuria. Unlike in lupus, in patients with nonlupus forms of glomerulonephritis, urinary MCP-1 was comparable to controls. CONCLUSIONS Altogether, the present data suggest a role for MCP-1 in mononuclear cell migration into the kidney in lupus nephritis.