Erdheim-Chester disease (ECD) is a rare form of non-Langerhansian histiocytosis classified by WHO since 2016 as a group L hematological neoplasia. Its pathophysiology is very complex and its multi-systemic character explains the plurality of clinical-radiological presentations. Bone involvement is almost constant while associated lung involvement is found in 50% of cases with non-specific clinical signs such as dyspnea and cough. Imaging provides an undeniable diagnostic contribution but confirmation of ECD is based on histopathology with immunohistochemical study. The search for the BRAF mutation by molecular biology has innovated the therapeutic management.The aim of our report is to relate a rare case of Erdheim-Chester disease in a 40-year-old man with an atypical pulmonary pseudotumor presentation without bone involvement and the diagnosis difficulty faced. Reporting of this type of aggressive disease should have an impact on early diagnosis and better prognosis.