Dean, A. G. (Pacific Research Section, NIAID, NIH, P.O. Box 1680, Honolulu, Hawaii 96806), and T. C. Jones. Seasonal gastroenteritis and malabsorption at an American military base in the Philippines. I. Clinical and epidemiologic investigations of the acute illness. Am J Epidemiol 95: 111–127, 1972.—Seasonal epidemics of acute gastroenteritis affecting 5,000–6,000 of 36,000 Americans occur annually at Clark Air Base. In the 1969 epidemic the illness was usually afebrile, with nonbloody diarrhea, abdominal cramps, constitutional symptoms, and sometimes vomiting. One-fifth of the patients had symptoms lasting more than 2 weeks, often with accompanying intestinal malabsorption. The epidemic occurred during the hottest season of the year but before the heaviest rainfall, and lasted 5 months. The incidence of disease was uniform on and off base, and among men, women and children. The secondary attack rate in families was not significantly greater than the overall attack rate. Among Americans, previous attacks or higher rank (greater age) were not protective, but local Filipinos working on the base had a significantly lower incidence. The illness was not confined to new arrivals. Sources of meals, recent trips, and consumption of water, soft drinks, milk and locally made beer did not seem causally related. The water supply appeared to have been well chlorinated and free of coliform bacteria. No microbiologic cause for the epidemic was found, but the suggestion is made that a waterborne, temperature-dependent agent may be responsible. Two hypothetical possibilities–blue-green algae and bacteriophage—are discussed.