‘Tree-in-Bud': Thinking beyond Infectious Causes

High-resolution computed tomography is an important diagnostic instrument in pneumology. The 'tree-in-bud' sign is a common finding in HRCT scans. The list of the most frequent differential diagnoses for 'tree-in-bud' sign includes infections with Mycobacterium tuberculosis, nontuberculous mycobacteria, and other bacterial, fungal, or viral pathogens. Other causes could be immunological, congenital, and idiopathic disorders as well as aspiration or inhalation of toxic agents. Rare differential diagnoses are malignant conditions. We present a case with 'tree-in-bud' pulmonary infiltrates of chronic lymphatic leukemia which was only diagnosed by transbronchial biopsies, and discuss other differential diagnoses.