Long-term studies of all types of primary glomerulonephritis (GN) taking into consideration the major morphological and clinical findings revealed the following: 1) Endocapillary GN, post-streptococcal type has a very good prognosis when only glomerulitis is present. The prognosis is significantly worse if either interstitial inflammation with fibrosis or nephrotic syndrome (NS) is present at the time of the biopsy. 2) The prognosis of the various types of mesangioproliferative GN (IgA nephritis, non-IgA nephritis, and immunohistologically negative GN) is very good if there is only glomerulitis. The prognosis is worse for all three types when the renal cortical interstitium exhibits inflammation with fibrosis at the time of the biopsy, and is worst of all when both interstitial fibrosis (IF) and the signs of acute renal failure (ARF) are present. Of this group, the type in which there are negative immunohistological findings exhibits the best prognosis. No difference in prognosis is found between IgA nephritis and non-IgA nephritis. 3) Minimal changes GN with NS has a very good prognosis when the interstitium is not involved. The presence of interstitial inflammation and fibrosis worsens the prognosis significantly. 4) Focal sclerosing GN has a much poorer prognosis than minimal changes GN with NS, even when there is glomerulitis only (5- and 10-year renal survival rates (RSRs) of 90% and 67%, respectively). If interstitial inflammation and fibrosis are present, the prognosis is significantly worse (5- and 10-year RSRs of 84% and 55%, respectively). The prognosis is worst when both ARF and IF are present at the time of the biopsy (5- and 10-year RSRs of 56% and 46%, respectively). From the clinical side, the prognosis is significantly worse if, at the time of the biopsy, NS is present or the serum creatinine concentration is elevated to more than 1.3 mg%. 5) Chronic membranous GN has a better prognosis than focal sclerosing GN if glomerulitis only is present (5-year RSR, 88%; 10-year RSR, 77%). If the renal cortical interstitium is also involved (in the form of IF), the prognosis is significantly worse (5-year RSR, 65%; 10-year RSR, 38%). The prognosis in this disease, too, is worst when both ARF and IF are present at the time of the biopsy (5-year RSR, 38%; 10-year RSR, 25%). 6) Membranoproliferative GN has a worse prognosis than any of the types of GN so far mentioned (5-year RSR, 51%; 10-year RSR, 32%).(ABSTRACT TRUNCATED AT 400 WORDS)