The renal biopsies from 123 patients who presented with proteinuria during pregnancy have been reviewed. Forty-seven showed glomerulonephritis that could be diagnosed on morphological grounds or on a clear-cut history of urine abnormalities before pregnancy. Fifty of the remaining 76 had been followed after pregnancy. In these 50 patients, a retrospective diagnosis of glomerulonephritis or preeclamptic toxemia was based on the persistence or disappearance of proteinuria and/or hematuria three or more months after pregnancy. Among 22 in whom a biopsy diagnosis of "pure" preeclamptic toxemia had been made, 19 showed disappearance of urine abnormalities after pregnancy, thus confirming the biopsy diagnosis. A retrospective analysis showed focal and segmental glomeris group but none in the preeclamptic toxemia group. Other differences could not be defined without quantitative evaluation; however, the glomeruli appeared to be more cellular in the glomerulonephritis group. The presence of IgA, IgG, IgM, Complement, and fibrin was not of value in distinguishing glomerulonephritis from preeclamptic toxemia.