To characterize demographic and regional variation in kidney stone prevalence in the U.S. we studied two nationwide cross-sectional surveys that included data on self-reported, physician-diagnosed kidney stones, supplementing published data on hospitalizations for stones. The larger study, Cancer Prevention Study II (CPS II), included 1,185,124 men and women, age > or = 30, recruited nationally in 1982, and provides state-specific prevalence estimates. The National Health and Nutrition Examination Survey (NHANES II) was a national probability sample of 25,286 U.S. adults interviewed between 1976 and 1980. Kidney stone prevalence increased with age until age 70, then declined and was higher in men than women and in whites than blacks. Prevalence among Hispanic and Asian men was intermediate between that of whites and blacks. There was a strong, statistically significant regional variability in stone prevalence among U.S. whites. The age-adjusted prevalence increased from north to south, and from west to east. The contrast in state-specific prevalence was greatest between men in North Carolina (prevalence = 14.9; 95% confidence interval = 14.2 to 15.7) and North Dakota (5.6; 4.7 to 6.4), and between women in South Carolina (6.4; 5.8 to 6.9) and South Dakota (2.4; 1.9 to 2.9). The marked variations in kidney stone prevalence by age, gender, race, and geographic location may provide clues to their etiology and prevention.