ObjectiveTo investigate clinical manifestations and epidemiological characteristics of patients with atypical pneumonia of unknown cause. MethodsClinical data and therapeutic effect of 11 cases of concentrated outbreak of atypical pneumonia with unknown cause were retrospectively analyzed.Results11 patients（3 males and 8 females，age 2232 years, average age 26.3 years）were admitted in the hospital, they worked at the same institution and occurred atypical pneumonia during the same period, the main clinical manifestations were fever, cough, expectoration, chest distress, but without difficulty in breathing. Computed tomography (CT) findings were as follows: increased bilateral lung markings, patchy and nodular shadows at single or multiple lobes, with blurred edge, small nodule shadow at mediastinum, no free liquid density shadow in the thoracic cavity; white blood cell count in peripheral blood was normal or decreased, detection results of common atypical pneumonia pathogens were negative. All hospitalized patients were isolated and given treatment, and were cured and discharged after receiving antiinfection treatment. ConclusionThis group of patients are with unknown etiology, atypical pneumonia occurs in an aggregated population, lung is characterized by patchy shadow, clinical symptoms are mild, prognosis is good after antiinfection treatment, and is in accordance with the characteristics of atypical pneumonia.