In May and June 1999, an outbreak of acute gastrointestinal illness occurred among long-distance hikers on the Appalachian Trail between Catawba and Troutville, Virginia. An investigation found that 45 out of 70 hikers had become ill within two days of arriving in Catawba, Virginia. Water samples were collected from a general store frequented by the hikers and from several nearby buildings and a popular all-you-can-eat restaurant. Symptoms were consistent with those caused by Norwalk-like viruses, and laboratory diagnosis detected Norwalk-like viruses in stool and serum specimens. People who consumed food items prepared at the general store were almost twice as likely to become ill as persons who did not consume those foods. Environmental sampling of water from the taps inside and outside the general store and from several surrounding establishments in Catawba found contamination by fecal coliform bacteria but not by Norwalk-like virus. Since several hikers reported illness prior to arriving at Catawba, person-to-person transmission of a highly contagious agent such as Norwalk-like virus could not be ruled out. Poor sanitation, scarce water supplies, and crowding can increase the risk of gastrointestinal illness among long-distance hikers.