There is little information available regarding traveler's diarrhea that affects a large number of Americans who differ widely in age and travel destination, and little or no information exists on self-treatment. This paper describes the clinical features, self-treatment, and outcome of diarrhea in 784 (95% follow-up) Americans who traveled for < or = 90 days, and who received detailed pre-travel advice. Diarrhea was reported by 46%. Of these, 34% (n = 270) had traveler's diarrhea (TD) defined as > or = 3 unformed stools/day +/- enteric symptoms or < 3 stools/day with > or = 1 enteric symptom, and 11% (n = 88) had loose motions (LM). Diarrhea was often severe and nearly a quarter of people with TD experienced fever and vomiting and 35% were required to alter their plans. The duration of travel and the destination itself were the strongest influences on diarrhea. Most travelers treated the illness themselves, whether or not they had LM (72%) or TD (83%). Those with LM took an antimotility agent or bismuth subsalicylate alone more frequently than those with TD (71% versus 48%, P < 0.002). Conversely, antibiotics were taken more frequently by those with TD (47% versus 27%, P < 0.03). Overall, 83% indicated self-treatment was effective (91% with LM and 80% with TD). Diarrhea is common despite pre-travel advice. Because travelers usually treat themselves, they should be provided with clear instructions on appropriate self-treatment.