Thoracic lymphangiomatosis (proliferation of anastomosing lymphatic vessels, of different sizes, in pulmonary, pleural and mediastinal regions) is an extremely rare disorder occurring mostly in childhood. We present a diffuse pulmonary lymphangiomatosis (DPL) case in a young adult female patient in which repeated surgical biopsies were inconclusive and transthoracic ultrasound-guided (TUS) biopsy led to the diagnosis. Even histologically, DPL is very difficult to differentiate from other lymphatic diseases such as lymphangioma and lymphangiomyomatosis, requiring an experienced pathologist and proper immunohistochemistry staining. This case highlights the importance of TUS-guided biopsies in the armamentarium of imagistic techniques in this very rare case.