We made an observational study of traveler's diarrhea in all members of a 48-person medical seminar group during a trip to Egypt and the Nile in September 1979. In addition to comparing symptoms in those who took antibiotic prophylaxis with doxycycline and those who did not, we also took into account water and dietary restrictions. Although our study supports the use of prophylactic doxycycline, the response appears to be considerably influenced by dietary or water restrictions. This study emphasizes the need to assess diet and water intake of travelers when conducting trials of medication aimed at preventing traveler's diarrhea.