In 1992, an outbreak of chronic diarrhea occurred among passengers on a cruise ship visiting the Galapagos Islands, Ecuador. Passengers (548) were surveyed, and stool and biopsy specimens from a sample who reported chronic diarrhea were examined. On completed questionnaires, returned by 394 passengers (72%), 58 (15%) reported having chronic diarrhea associated with urgency (84%), weight loss (77%), fatigue (71%), and fecal incontinence (62%). Illness began 11 days (median) after boarding the ship and lasted 7 to >42 months. Macroscopic and histologic abnormalities of the colon were common, but extensive laboratory examination revealed no etiologic agent. No one responded to antimicrobial therapy. Patients were more likely than well passengers to have drunk the ship's unbottled water or ice before onset of illness and to have eaten raw sliced fruits and vegetables washed in unbottled water. Water handling and chlorination on the ship were deficient. Outbreaks of a similar illness, Brainerd diarrhea, have been reported in the United States. Although its etiology remains unknown, Brainerd diarrhea may also occur among travelers.