With the advent of rapid and convenient means of transportation, millions of persons travel each year from industrialized to developing countries in the tropics and subtropics. These travelers are at risk for a variety of infectious diseases that are endemic in these areas; the most frequently occurring of these is diarrhea. Studies of groups of travelers to Latin America and Africa have found that approximately one-half develop diarrhea during their stay abroad. Etiologic investigations of these illnesses have demonstrated that the important agents that cause travelers' diarrhea are similar to those that cause diarrhea in children in the developing countries. One-third of the cases are associated with enterotoxin-producing strains of Escherichia coli. Smaller proportions appear to be due to rotavirus, Norwalk virus, Shigella, Salmonella, Giardia lamblia, and Entamoeba histolytica. Although they have not been fully evaluated in travelers' illnesses in Latin America or Africa, Campylobacter jejuni, Aeromonas hydrophila, other viruses, and Cryptosporidium probably cause some of the currently unexplained cases of diarrhea.