The frequencies of kidney and urinary tract diseases (KUTD) in a defined population are presented. Rates of first hospitalizations with KUTD in the 5-year period, 1971 through 1975, were determined among over 1 million members of the Kaiser Foundation Health Plan (KFHP) of Northern California. Cases were identified from computer-stored ICDA-coded final discharge diagnoses for hospitalizations. Kidney and other urinary tract diseases were final diagnoses in 35.7 first hospitalizations per 10,000 members per year in the period 1971 through 1975. Disease-specific rates were highest for urinary tract infections, benign prostatic hypertrophy, and renal and ureteral calculi. The frequency of KUTD in this population is compared to available rates from other sources.