OBJECTIVE To describe a patient with human chorionic gonadotropin (hCG)-secreting giant cell carcinoma of the lung manifesting as painful gynecomastia, which resolved after surgical treatment. METHODS We present the clinical, radiologic, and clinical course of our patient, who presented with painful gynecomastia. RESULTS A 51-year-old man presented with a lung mass and rapidly progressing bilateral painful gynecomastia. Hormonal evaluation revealed high levels of hCG beta subunit, estradiol, and total and free testosterone and suppressed levels of luteinizing hormone and follicle-stimulating hormone. The patient underwent resection of the right upper and middle lobes of the lung. After the surgical procedure, the levels of hCG and estradiol rapidly decreased, and the gynecomastia resolved completely within 3 months. Fluctuation in the levels of serum hCG paralleled the clinical course of the lung tumor. CONCLUSIONS Although gynecomastia is a common disorder, hormonal work-up including hCG beta subunit should be done in an adult male patient presenting with rapidly progressing or recent onset of painful gynecomastia. Lung cancer should be considered in the differential diagnosis of gynecomastia attributable to ectopic production of hCG beta subunit.