All five siblings (three boys and two girls, aged 1.5-9 years) in a family developed hemolytic-uremic syndrome associated with verotoxin-producing Escherichia coli O157:H7 at a lakeside vacation cottage during the fall of 1985. All five were hospitalized and made a full recovery. Both parents remained asymptomatic, and neither had evidence of this infection. In four children who were investigated prospectively, free verotoxin was still detectable in the stools for between 3 and 7 weeks. The prodromal diarrheal illness in the children occurred over a 10-day period. The epidemic curve was consistent with a point-source outbreak, but continuous exposure or person-to-person transmission could not be ruled out. The source of the infection was not identified.