In June 1983, an outbreak of waterborne giardiasis occurred in a group of 93 university students and faculty participating in a geology field course in Colorado. All cases occurred in one subgroup of persons who were heavily exposed to untreated stream water on a field trip, and the risk of illness was strongly related to the amount of untreated stream water consumed. The median incubation period from a brief exposure to the first symptom was 7 days. The authors compared symptoms and stool sample results among 31 Giardia-positive persons in the exposed group and 36 Giardia-negative participants in an unexposed group to assess several case definitions for acute giardiasis. Diarrhea, abdominal cramps, flatulence, foul-smelling stools, nausea, excessive tiredness, bloating, anorexia, and chills were each significantly more common in the first group than in the second. A giardiasis case definition of 5 days or more of diarrhea—the definition used in many epidemiologic studies of giardiasis—had a specificity of 100 percent but a sensitivity of only 32.2 percent compared with a definition based on results of stool examinations. When a case was defined as an illness lasting 7 days or more, with a combination of two or more of six symptoms (diarrhea, flatulence, foul-smelling stools, nausea, abdominal cramps, and excessive tiredness), sensitivity rose to 73 percent, with a specificity of 88 percent. Such a case definition may be an improvement over that of 5 days of diarrhea, especially in outbreaks where there is good laboratory documentation that Giardia is the etiologic agent. The definition should be validated in other outbreaks and in situations where giardiasis must be distinguished from gastrointestinal disease caused by other agents. Am J Epidemiol 1991;133:402–7.