The need to treat asymptomatic adults who have intestinal parasites should be based on considerations of potential pathogenicity and transmission risk, but such data are sparse in general and unavailable for countries such as the United States, where reinfection is rare. Opinions of North American physicians about the natural history and treatment of 12 parasites detectable by stool examination were assessed using the Delphi technique. In response to the first questionnaire, majority agreement on pathogenic potential was achieved for only 3 of 12 parasites. When the more expert group was queried a second time, majority agreement was achieved for 8 of 12 parasites. Nevertheless, the predicted risk of subsequent disease still varied widely, from 1:10 to 1:1,000 for most parasites, and was unrelated to the perceived need for treatment. Almost all respondents thought asymptomatic adults with any intestinal parasites should always be treated. Transmission risk was judged to be low and was generally appropriate to the life cycle of the parasite.