Percutaneous renal biopsies from 56 patients with systemic lupus erythematosus were studied to determine the relationship between renal function and the light microscopic, electron microscopic and immunofluorescent antibody findings. The glomerular lesions were classified into 5 major groups: diffuse membranoproliferative glomerulonephritis (34%), diffuse proliferative glomerulonephritis (26.8%), membranous nephropathy (12.5%), minimal lesion ('nil') and minimal lesion with increased mesangial matrix and/or cells (21.4%) and focal and segmental glomerulonephritis (5.3%). Minimal lesions and focal and segmental glomerulonephritis were invariably associated with normal renal function. Patients with moderate to severe renal involvement and the nephrotic syndrome had predominantly diffuse membranoproliferative and diffuse proliferative glomerulonephritis. Membranous nephropathy was associated with moderate renal involvement and the nephrotic syndrome in 50% of cases. Patients with pure mesangial electron dense deposits had normal renal function or mild renal involvement when the deposits were heavy. Moderate and heavy subepithelial, and intramembranous/subepithelial deposits were associated with moderate to severe renal involvement and the nephrotic syndrome. Renal involvement was most severe with heavy subendothelial deposits. Cytoplasmic tubuloreticular structures measuring approximately 18 to 20 nm in diameter and 80 to 100 nm in length were found in 93% of all biopsies, but bore no relationship to the renal function of the patients. Anti-Hu-IgG fluorescent deposits were found in all the renal biopsies; in 81.3% these were associated with less heavily stained deposits of immunoglobulin IgA, IgD and IgM. Early complement components Clq and C4 were utilized in the complement pathway of activation. Pure mesangial fluorescent deposits were associated with normal renal function or mild proteinuria. Diffuse granular and lumpy deposits along the capillary loops were usually associated with moderate to severe renal involvement and the nephrotic syndrome. In the present series of cases, there was a good correlation between renal function of patients with systemic lupus erythematosus and the glomerular lesions as determined by light, electron microscopic and immunofluorescent microscopic findings.