In July 2012, the U.S. Naval Medical Research Unit No. 6 investigated an outbreak of gastrointestinal illness characterized by diarrhea among U.S. service members participating in Operation New Horizons in Pisco, Peru. Overall, there were 25 cases of self-reported diarrheal illness among 101 respondents to a questionnaire (attack rate: 24.8%). Personnel who consumed food that was prepared at the two hotels where they were lodged were more likely to report diarrhea than those who did not eat at the hotels (40.9% [9/22] versus 20.3% [16/79]; RR=2.1; p=.047). The difference in diarrhea attack rates between lodgers at the two hotels was not statistically significant. Known or putative pathogens were identified in 72.7 percent (8/11) of samples tested: Blastocystis hominis, Shigella sonnei, diffusely adherent Escherichia coli, and norovirus genotypes I and II. The investigation's findings suggested a food-borne etiology from hotel kitchens. Among all personnel, hand-washing hygiene was reinforced; however, food sources were not restricted.