Background  Legionella pneumonia caused by Legionella pneumophila is often characterized by severe and potentially fatal pneumonia with high mortality. The incidence has been increasing in recent years. Because of its atypical clinical and imaging manifestations, it is easy to cause misdiagnosis and underdiagnosis.

  Objective  To improve the level of diagnosis and treatment of Legionella pneumonia by summarizing the clinical characteristics and outcome of the disease.

  Methods  Clinical data about 9 patients with diagnosed Legionella pneumonia admitted to the First Medical Center of Chinese PLA General Hospital from March 2019 to October 2021 were retrospectively analyzed, and the clinical features, imaging findings, treatment and outcome were analyzed.

  Results  Among the 9 cases, 6 cases were male and 3 cases were female, with a median age of 66 (25-82) years. They were diagnosed by sputum, bronchoalveolar lavage fluid and / or blood metagenomic next generation sequencing, urinary Legionella antigen, Legionella antibody and so on. All the patients had an acute onset. The clinical symptoms included hyperpyrexia (9/9), cough (9/9), dyspnea (9/9), diarrhea (6/9), fatigue (5/9), and multisystem involvement (9/9). C-reactive protein and interleukin-6 were elevated in all cases, and procalcitonin was significantly elevated in 7 cases. Blood potassium decreased in 8 cases, and phosphorus decreased in 6 cases. The chest images showed rapid progression, and the chest CT showed patchy density-enhancing shadow or consolidation, surrounded by ground-glass opacity, in which bronchial inflation sign and pleural effusion could be seen. Macrolides and neo-fluoroquinolones were the first-line therapy. Eight cases were discharged with improvement and one case died.

  Conclusion  The clinical and imaging manifestations of Legionella pneumonia are atypical. Patients who suspected to be community-acquired pneumonia with multisystem symptoms, along with hyponatremia, hypokalemia, hypophosphatemia, abnormal infection-related tests, and no response to β-lactam antibiotics should be alerted to the possibility of legionella pneumonia, and early targeted examination and treatment may improve the prognosis.