Light and immunofluorescent microscopic patterns of lupus nephritis in 203 biopsies, 1 nephrectomy, and 20 autopsies from 179 patients were analyzed. The latest World Health Organization (WHO) classification was used. Seventy patients had diffuse lupus nephritis, 43 mesangial, 19 membranous, 19 focal, and 16 minimal change; 2 patients had advanced sclerosing nephritis. Nine patients were difficult to classify by light microscopy, but 3 of these could be classified with the help of immunofluorescence microscopy. Strict definition, especially of category III (focal and segmental lupus nephritis) is important, since this lesion has a tendency to heal, and patients with few immune deposits outside the segmental lesions have a rather good prognosis. Category V (membranous lupus nephritis) should probably be limited to membranous lesions with pure subepithelial deposits or with subepithelial and mesangial deposits, while membranous changes associated with diffuse or focal proliferative lesions are better classified as Category IV (diffuse lupus nephritis). It was observed that steroid treatment reduces the amount of deposits, especially those in the subendothelial and mesangial locations. The amount of proliferation is also reduced, but in a considerable proportion of cases, it is replaced by sclerosis. Therefore, interpretation of biopsy patterns must take prior therapy into consideration. Immunofluorescence findings in the glomeruli correlated quite well with light microscopic patterns. Active interstitial inflammation, which is most common in diffuse lupus nephritis, was only observed in the presence of tubulointerstitial immune deposits. Acute arteritis was much more common in autopsy than in biopsy specimens pointing to its ominous nature. It was concluded that combined examination of biopsies by light and immunofluorescence microscopy as well as electron microscopy, and strict categorization of lesions are valuable diagnostic and prognostic aids. Their usefulness is considerably enhanced if certain clinical data, such as prior therapy are taken into consideration.