A 74-year-old male with a history of chronic lymphocytic leukemia (CLL) previously treated with fludarabine/cyclophosphamide/rituximab (FCR) 5 years ago, presented with progressive fatigue, mucocutaneous bleeding, and cytopenias (hemoglobin 51 g/L, platelets 8.0 × 10⁹/L, lymphocytes 0.4 × 10⁹/L). He had normal respiratory findings, and no lymphadenopathy or hepatosplenomegaly. Further workup revealed a small spiculated lung nodule and multiple sclerotic bony lesions. Due to bleeding/profound thrombocytopenia, lung biopsy was not feasible. Peripheral smear revealed leukoerythroblastosis with few nucleated red blood cells and left shift of granulocytes. Bone marrow (BM) aspirate yielded a dry tap with clusters of extrinsic atypical cells on touch preparations. BM core biopsy showed infiltration and near complete replacement by a population of highly atypical cells with surrounding fibrosis. Cells were positive for cytokeratins CK7 and CK8/18, Napsin A, and thyroid transcription factor-1, specific for a primary poorly differentiated lung adenocarcinoma. Leukoerythroblastosis in association with cytopenia often indicates a BM infiltration and warrants an early BM biopsy to rule out hematological and solid malignancies, particularly in CLL patients treated with FCR. In our case, a diagnosis of a lung adenocarcinoma was established by BM examination, the only clinically feasible diagnostic modality.