The use of contaminated, untreated or inadequately treated groundwater was responsible for 51 percent of all waterborne outbreaks and 40 percent of all waterborne illness reported in the United States during the period 1971-82. Contaminated, untreated or inadequately disinfected groundwater caused 65 percent of the waterborne outbreaks and 66 percent of the waterborne illness which occurred in noncommunity and individual water systems but only 32 percent of the outbreaks and 31 percent of illness in community water systems. Illnesses most frequently transmitted through groundwater included acute gastroenteritis of undetermined etiology, chemical poisonings, hepatitis A, shigellosis, and viral gastroenteritis. Waterborne outbreaks in water systems using untreated well water were caused primarily by the overflow or seepage of sewage from septic tanks or cesspools, chemical contamination, and surface runoff contamination. An increase in the number of outbreaks resulting from the use of untreated, contaminated groundwater was noted during the summer months.