An outbreak of E. coli O121 infections was associated with swimming in a Connecticut Lake. Serologic testing for seven case-patients showed high antibody titers to E. coli O121. Three children developed hemolytic uremic syndrome as a result of their illness. E. coli indicative of fecal contamination was identified from sediment and water samples taken from a storm drain that emptied into the beach area and from a stream bed located between 2 houses, but no Shiga toxin–producing E. coli was ever found.