Population-based incidence data for IgA nephropathy (IgAN) are available for some countries but not for the United States. The purpose of this study was to determine the incidence of IgAN in central and eastern Kentucky for 5- and 10-yr periods between 1975 and 1994 and to examine differences among patient groups between those periods. The incidence of IgAN was 5.4 cases per one million population per year (MPPY) for period 1A (1975 through 1979), increasing to 12.4 cases per MPPY for period 2B (1990 through 1994) (P < 0.001). Males had a 2.7 times higher incidence than females for period 1 (1975 through 1984) and 2.2 times higher for period 2 (1985 through 1994). For period 1A, the incidence for Fayette County, which includes the city of Lexington, was lower than that of the rest of the study area (P=0.26), whereas for period 2 the incidence was higher for Fayette County (P=0.052). During period 1, the highest incidence of IgAN for any age and gender group was 24.3 cases per MPPY for males ages 30 through 39. For period 2, the incidence for males was similar for each decade between ages 20 and 59 (approximately 19 cases per MPPY). No African-American was diagnosed during period 1, but in period 2 incidences for blacks and whites were similar (10.7 and 10.2 cases per MPPY, respectively). For the last 5 yr of the study (1990 through 1994), the incidence of end-stage renal disease (ESRD) due to IgAN was 5.5 cases per MPPY: 8.4 for males and 2.7 for females. The incidence of IgAN in Kentucky for period 2B was still much lower than that in European studies, but the incidence of ESRD due to IgAN may be similar. Thus, IgAN may be as important a condition with respect to ESRD in Kentucky as it is in other regions of the world.