On October 31, 2004, a stream overflowed, flooding the University of Hawaii (UH) campus. On November 19, 2004, a possible flood-related leptospirosis case (Patient 1) was reported to the Hawaii State Department of Health (HDOH). Surveillance for febrile illness was established through an Internet questionnaire. Active case finding was conducted among groups involved in the flood clean-up. Free leptospirosis testing was offered by HDOH. Patient 1's illness was confirmed as leptospirosis by microscopic agglutination testing. A total of 271 persons responded to the Internet survey, of whom 90 (33%) reported a febrile illness within 30 days of contact with flood water. Forty-eight respondents (18%) were tested for leptospirosis. One additional acute leptospirosis case was identified. Patient 2 was epidemiologically linked to Patient 1. Health care providers should maintain a high level of suspicion for leptospirosis after flooding events, and local public health officials should promote leptospirosis awareness among flood-affected populations.