• An acute gastrointestinal tract illness affected 213 (52%) of 407 campers and 64 (53%) of 121 staff members attending a boys' camp in the Catoctin Mountains of Maryland during the summer of 1981. Nausea was the predominant symptom for ill campers and staff members (73%), but more staff members experienced diarrhea (49%) than did campers (9%). Twenty-three individuals had more than one episode of illness compatible with the case definition. Eight of nine paired blood specimens from ill staff members showed a fourfold increase in antibody titer to Norwalk virus by radioimmunoassay. Environmental inspections and laboratory tests failed to implicate a common source of exposure.