We reviewed the initial serological data of 50 patients with biopsy-proven lupus nephritis. As compared with a group of lupus patients without nephritis, patients with nephritis had lower serum complement C3 (p less than 0.05) and C4 (p less than 0.005) levels and higher serum DNA binding activity (p less than 0.001). The frequency of rheumatoid factor, antiphospholipid, anti-ENA, and fluorescent antinuclear antibodies was similar in both groups. We correlated the serological data of the patients with nephritis with the clinical severity of their disease. Using a functional staging system based on the serum albumin and creatinine levels at the time of biopsy, we found that patients with functionally milder disease (proteinuria without nephrotic syndrome or renal failure) had higher C3 (p less than 0.05) and lower DNA binding (p less than 0.005) than patients in the more severe functional classes (nephrotic syndrome with or without renal failure). In contrast, C4 levels were always very low, irrespective of functional severity. We also correlated the serological data with the pathological findings. Patients suffering from diffuse proliferative nephritis had higher DNA binding values than patients with focal proliferative (p less than 0.01) or membranous (p less than 0.001) nephritis. By contrast, complement levels were not correlated with the severity of biopsy changes. Taken together, the data presented here suggest that C3 and DNA binding, but not C4, correlate with the clinical severity of lupus nephritis at presentation whereas DNA binding, but not complement levels, correlates with the severity of pathological changes.