A 65 year old male smoker was diagnosed with squamous cell carcinoma of upper lobe of the right lung complicated with Horner's syndrome and gradually increasing leucocytosis. Alhough the inflammatory biomarker level in serum was low, there was no definite way to determine the cause of the leucocytosis (whether infection or hematologic paraneoplastic syndrome). After empirical antibiotic therapy, his fever subsided but the leucocytosis persisted. It was difficult for us to take a decision regarding the priority of the treatment of infection or the lung cancer. Only after the first cycle chemotherapy, did the leucocytosis rapidly drop down. Normal serum procalcitonin level and quick response to chemotherapy indicated that leucocytosis was a manifestation of paraneoplastic syndrome. Treating the underlying cancer is the first step.