Hypercalcaemia and leukocytosis are two paraneoplastic conditions associated with poor prognosis. Adenosquamous carcinoma is a rare and aggressive histological subtype of lung cancer consisting of adenocarcinoma and squamous cell components. We report the case of a 57-year-old male smoker who was admitted to the Emergency Room with skull and neck tumefactions, confusion and deteriorated general condition. The complementary study in the ER revealed severe hypercalcaemia (19.8 mg/dL), leukocytosis (18.7 × 109/L) and extensive osteolytic lesions of the skull on cranioencephalic computer tomography (CT). The patient was stabilized and admitted. Thoracoabdominopelvic CT showed lung parenchyma consolidation with necrotic areas, supra and infradiaphragmatic adenopathies and scattered osteolytic lesions. Percutaneous lymph node biopsy was consistent with metastasis of adenosquamous lung carcinoma. The patients' clinical situation evolved unfavourably after hospital-acquired infection. This case is characterized by a rare presentation of advanced stage adenosquamous lung carcinoma with scattered osteolytic lesions and severe hypercalcaemia-leukocytosis syndrome, an underrecognized marker of poor prognosis.