Field and clinical studies of endemic goiter in Northern Virginia during 1965 and 1966 are summarized. The hypotheses tested were (a) that a goiter endemia would be found in a rural population served by shallow wells and (b) that, in this type of population, goiter prevalence would be higher among those served by shallow wells than in a similarly constituted population served by a protected public supply. Richmond County, Va. was selected (in consultation with the Virginia State Department of Health and the Region III Office, USPHS) because of its rural population, high water table and the well-protected public supply in Warsaw, Va., serving approximately 10% of the population. The entire school population of the county (1238 children) as well as 50 families in each of 2 hamlets were surveyed for thyroid enlargement using a technique that minimized examiner bias. Private water supplies and the general sanitation of rural families were surveyed by one of the investigators; dietary iodine intake by chemical assay of food composites was determined and clinical studies of thyroid function were done in 11 goitrous boys using 132I. Goiter prevalence was 29% in the school population and slightly higher (but not significantly so) in the Negro than in the white population, apparently due to an excess among Negro females aged 10–14. Goiter prevalence in the 5–9 age group (25%) was the same in both races and the same in males as in females. In the 2 communities surveyed, goiter prevalence in Negroes of all ages and in both sexes was higher than in the white population. Radioisotope studies of 11 goitrous boys yielded no evidence of iodine deficiency; 3-hr uptakes, thyroidal I clearances and plasma inorganic I (PII) concentrations were within normal limits. Dietary I intakes were higher among adult white women (384 μg/day) than among Negro women (165 μg/day) but were the same in goitrous and nongoitrous women. Goiter prevalence was significantly higher (.01 E. coli on analysis by standard methods. The results supported the hypotheses tested and suggested that unprotected water supplies may exert a goitrogenic effect—an effect more marked in children less than 10 yr old. There was no evidence of nutritional I deficiency; nor were other stigmata of endemic goiter observed. No evidence as to the nature of the goitrogen was adduced; but intermittent pollution, probably of the entire high water table, could not be excluded.