In patients with lupus nephritis, progression from a mild lesion to a diffuse proliferative glomerulonephritis has been reported in one to 35 percent of patients. Because of the wide variation in the rate of progression, this study was undertaken to determine those factors which would identify the patients most likely to progress. Of 21 patients with a mild lupus nephritis by light microscopy, progression to a diffuse proliferative lesion was seen in only those patients who had subendothelial deposits. While not all patients with subendothelial deposits had a deteriorating course, the persistence of such deposits on subsequent biopsies indicated a poor prognosis.