Between July 7 and 18, 1991, five boys from a small town in rural Illinois experienced the onset of an acute febrile illness subsequently confirmed as leptospirosis by serologic tests. A cohort study found that swimming in a small swimming hole, Steel Tunnel Pond, was associated with disease (P < 0.01), the attack rate being 28%. Leptospira interrogans serovar grippotyphosa was isolated from urine cultures from two of the case patients and from a culture of Steel Tunnel Pond water. A high seroprevalence for grippotyphosa was found in animals near the pond. Drought conditions had been present in the month before the outbreak, creating an environment in the pond which probably facilitated transmission of the organism from area animals to humans. Although leptospirosis is infrequently reported in humans in the United States, it is endemic in animals and the potential for outbreaks exists, especially when environmental conditions are favorable.