To investigate diarrhea in tourists on a worldwide scale, 16,568 passengers were interviewed during their flights home from 13 destinations in various climatic regions. Significant differences in diarrheal incidence were observed between individual destinations and also between hotels in the same area. The highest rates exceeded 50%. Younger persons were more often affected. Sex, travel characteristics, and a record of former tropical journeys influenced the incidence to a minor degree. Even in the tropics, diarrhea usually takes a short (average, 3.6 days) and mild (average of 4.6 stools per day) course. Prophylactic or therapeutic medication only slightly influenced these values. The various regions showed minor differences in chronology and symptomatology. This is consistent with quantitative rather than qualitative geographic variations in causative agents. The traditional nutritional recommendations for prophylaxis seemed to be unrealistic and usually unsuccessful.