An outbreak of infectious diarrhea with 70 laboratory-confirmed cases (58 w
ith Giardia lamblia) and 107 probable cases occurred in U.K. tourists who s
tayed in a hotel in Greece. After a cluster of six cases in persons who had
stayed at the hotel was reported, the Communicable Disease Surveillance Ce
ntre began active case ascertainment. This outbreak illustrates the value o
f an approach to surveillance that integrates routine surveillance data wit
h active case ascertainment.