Studies performed at large metropolitan medical centers have reported an increasing incidence of idiopathic focal segmental glomerulosclerosis (FSGS) in adults. To determine whether a similar trend occurs in small urban and rural communities and to determine the role of race in these observations, we reviewed the patient records of all adults who underwent renal biopsies at our institution over the 20-year period from 1974 to 1994. The patients were grouped for analysis in 5-year intervals, 1975 to 1979, 1980 to 1984, 1985 to 1989, and 1990 to 1994, for the following diagnoses: FSGS, membranous nephropathy (MN), minimal change nephropathy (MCN), membranoproliferative glomerulonephritis (MPGN), immunoglobulin A (IgA) nephropathy, chronic glomerulonephritis, diabetic nephropathy, hypertensive nephrosclerosis, and chronic interstitial nephritis. Patients with secondary causes for these lesions were excluded. The relative frequency of FSGS increased from 13.7% during 1975 to 1979 to 25% during 1990 to 1994 (P < 0.05). The relative frequency of MN decreased from 38.3% during 1975 to 1979 to 14.5% during 1990 to 1994 (P < 0.01). There were no changes in the frequencies of MCN, MPGN, IgA nephropathy, chronic glomerulonephritis, diabetic nephropathy, hypertensive nephrosclerosis, or chronic interstitial nephritis over the 20-year period. However, there was a significant increase in the percentage of blacks with FSGS, from 0% in 1975 to 1979 to 22.6% in 1990 to 1994, and an increased percentage of Hispanics with FSGS, from 0% in 1975 to 1979 to 21.3% in 1990 to 1994 (P < 0.05). The modest increase in whites with FSGS did not reach statistical significance. The incidence of MN in blacks and whites decreased over the 20-year period. In the last 5 years, 15 patients per year had FSGS compared with 7 patients per year with MN (P < 0.05). No changes in age or sex between groups or over time accounted for these results. We conclude that FSGS is now diagnosed twice as often as MN and is the most common idiopathic glomerular disease at our hospital. Reasons for this increase include the emergence of FSGS in both Hispanics and blacks, with a modest increase of FSGS in whites. The increase in FSGS in the three most common races in our community suggests that factors other than genetic, perhaps environmental, have a role in the pathogenesis of FSGS.