A high incidence of diarrhea was reported in a group of approximately 1,400 Americans who traveled to the Portuguese island of Madeira in October 1976. A mail questionnaire survey revealed that 39% of the responding 859 travelers experienced diarrhea; in 42% of these diarrhea lasted for longer than 1 week. The most frequent accompanying symptoms were abdominal cramps (75%), abdominal distention (72%), nausea (70%), and weight loss (40%). Of all travelers surveyed, 33% developed an illness resembling giardiasis with a median incubation period of 4 days. Of 35 ill patients who had a stool culture, enteric pathogens were recovered from 4 (3 Shigella and 1 Salmonella). On the other hand, of 58 ill patients whose stools were examined for parasites, Giardia lamblia was recovered from 27 (47%). Analysis of the epidemiologic data showed that drinking tap-water on the island was significantly associated with illness; eating ice cream or raw vegetables on the island was also implicated. There was no evidence of continuing transmission of giardiasis in American tourists visiting Madeira 8--12 months after the outbreak.