Legionnaires' disease became widely known following an outbreak of pneumonia in the United States in 1976. It is often caused by infection from artificial water sources such as cooling towers, water supply and heating systems, and recirculating hot tubs. To effectively implement infection prevention measures for Legionnaires' disease, collaboration among healthcare workers, water supply and heating system managers, building hygiene personnel, and other relevant parties is essential. It is important to note that outbreaks of Legionnaires' disease continue to occur frequently both domestically and internationally. While the number of reported cases of Legionnaires' disease in Japan has increased, the mortality rate has decreased but has stabilized at a lower level. Caution is also required as reports have been made in association with disasters and travel, in addition to artificial environmental water.

We present a case of a critically ill patient with severe Legionella pneumonia complicated by morbid obesity (BMI ≥ 40 kg/m²) who was successfully treated with early initiation of veno-venous ECMO (V-V-ECMO) without any sequelae. The patient, a 48-year-old male, initially presented with symptoms of a sore throat, fever, significant fatigue, and decreased appetite. Upon diagnosis of severe pneumonia complicated by morbid obesity, he was transferred to our hospital for further management. Upon admission, he was promptly intubated and placed on mechanical ventilation. Due to a positive urinary Legionella antigen test indicating a risk of deterioration, V-V ECMO was initiated immediately after intubation. During ECMO support, the patient received Levofloxacin Hydrate at 500 mg/day and Prednisolone Sodium Succinate at 100 mg/day. He was successfully weaned off ECMO after 12 days and transferred back to the referring hospital on day 20. While ECMO therapy for morbidly obese patients was traditionally considered relatively contraindicated, this case suggest that obesity alone is not a contraindication to initiating ECMO.