A 54-year-old female resident of Wellston, Okla, was found to have osteosclerosis on a routine chest roentgenogram. Subsequent investigation disclosed the cause of her osteosclerosis to be fluorosis secondary to the ingestion of well water containing 429 mumol/L of fluoride (recommended levels, 11 to 58 mumol/L). Water samples were also obtained from the 12 wells on properties adjacent to the index case. In three other wells, all at similar depths as the well of the index case, the fluoride concentration of the water was greater than 212 mumol/L. Urine samples from members of the four households who obtain their drinking water from these wells contained elevated urinary fluoride levels. Thus, fluorosis may develop in certain areas of the United States as a result of the natural occurrence of fluoride in the groundwater. Consequently, in known endemic areas, it would appear reasonable to measure the fluoride concentration of the well water at the time of drilling.