On July 14, 1998 [corrected], the Wisconsin Division of Health (WDOH) was notified by the City of Madison Health Department that three athletes were hospitalized with an acute febrile illness. The illness was characterized by fever, myalgia, and headache with illness onset on July 6, 7, and 10, respectively. One of these three athletes had acute renal failure. Two of the athletes had participated in a triathlon held in Madison, Wisconsin, on July 5 (692 registered participants) and all three had participated in a June 21 triathlon in Springfield, Illinois (961 registered participants). Eighty persons were registered for both events. Leptospirosis was suspected by WDOH staff as a likely cause of the illness and CDC was notified. Acute-phase serum specimens from two of the three hospitalized athletes obtained 4 and 8 days following onset of fever have been tested at CDC for leptospirosis using the PanBio enzyme-linked immunosorbent assay (ELISA) IgM screening test (PanBio, Brisbane, Australia); one specimen tested positive. This report presents preliminary findings of an ongoing investigation to identify additional cases of acute febrile illness among athletes participating in these two triathlons and to determine the cause of the illness.