Objective To explore the diagnosis and treatment process, clinical manifestation and outcome of lung cancer patients with hyponatremia as the first symptom. Methods Retrospective analysis of 4 patients with hyponatremia. Results Four patients with lung cancer were first diagnosed with hyponatremia, their serum sodium was between 112~122mmol/L, and their plasma osmotic pressure was low and urine osmotic pressure was high, so they were diagnosed as SIADH. The treatment of water restriction and sodium supplement can significantly increase the concentration of serum sodium. Through follow-up, we know that 3 patients with primary disease are in stable condition after active treatment, but the other one patient has died. Conclusion The clinical manifestation of SIADH, caused by lung cancer is hyponatremia. This provides clues for the diagnosis of lung cancer. However, the incidence of the disease is hidden, if early detection and active treatment will help to improve the prognosis of patients.