The rate of occurrence of stones of the urinary tract was assessed in a large population served by the Northern California Kaiser Foundation Health Plan. The study involved three separate groups. First, data were obtained by questionnaire from approximately 175,000 adults who took a multiphasic health checkup in the period 1964-1972; of these generally well adult members, 26.2/1000 persons (32.0/1000 men and 21.0/1000 women) reported having ever been told by a physician that they had a urinary tract stone. Second, data were obtained from 139,000 persons served by the San Francisco outpatient facility in 1970-1972; 1.22/1000 per year (1.81/1000 men and 0.59/1000 women) had an initial diagnosis of a "new or recurrent" stone of the upper urinary tract. The third set of data was procured from the entire Northern California region in 1971-1975; 0.36/1000 (0.52/1000 men and 0.19/1000 women) were discharged from a hospital each year with a diagnosis of upper urinary tract stone. All rates were age-adjusted to the 1960 US Census population. Of these three rates, the rate derived from the outpatient visit record most closely estimates incidence, since nearly all persons who are hospitalized are first seen as outpatients. Rates of kidney stone diagnosis were three times more common in men and, although rare before 20 years of age, the frequency increased rapidly and peaked in the age group 40 to 59 years. Rates were approximately twice as high in whites as in blacks and Orientals; the frequency of stones was inversely related to socioeconomic status as measured by level of education. Over 90% of stones occurred in the upper urinary tract, and the majority contained calcium oxalate. Population-based rates of occurrence of kidney stones are not generally available in the United States. Comparisons with the few available studies indicate that rates in the Kaiser Foundation Health Plan population may be high.