In August 1994, an epidemic of acute febrile illness occurred at the Education Center Building of a company in Shibuya-ku, Tokyo. All 43 trainees attended in two groups and 2 staff members of the Center fell ill. The 45 patients came to one of our hospitals in two groups, and 35 patients were treated. The patients were 4 males and 31 females, and the average age was 29.0 years. The duration until falling ill was 36 to 90 hours after entering the Center. Symptoms were fever, lumbago arthralgia, headache, dyspnea, general fatigue, etc. Physical examination revealed slightly injected mucosa of the pharynx in a patient who complained of a sore throat. On laboratory examination, leukocytosis with a left shift of the nucleus and elevation of serum CRP levels were found. Erythromycin (600 mg, daily) and nonsteroidal antiinflammatory drugs (NSAIDs) were given by mouth to almost every patient. Two patients were hospitalized. The illness was self-limited, generally lasting from two to five days. Strains of legionellae isolated from the water of the cooling tower located at the top of the Center, were identified as L. pneumophila serogroup 7. Since seroconversion in a patient against the cooling tower strain from 1:16 to 1:256 was determined and the clinical courses agreed with the definition of Pontiac fever by Glick et al, we concluded that the epidemic was an outbreak of Pontiac fever due to L. pneumophila serogroup 7. Pontiac fever is considered to be one of the community-acquired diseases. Thus, we have to note that Pontiac fever may be misdiagnosed as we examine patients who complain of the symptoms noted above.