The lung is the most common site of cancer in the world today and it has a poor survival rate. The best way of reducing the lung cancer occurrence is giving up the habit of smoking. It is necessary to establish routine ways of early diagnosis according to clinical symptoms and chest radiography in general practice. It is said to be a main key of right timing of the adequate therapy. In a minority of patients with tumours, signs and symptoms develop that cannot be explained on the basis of either the mass effect produced by the primary tumour (or its metastases) or the production of a hormone normally associated with the tissue type that has given rise to the malignant tumour. These are known as paraneoplastic syndromes and may cause various symptoms. They may lead also to hyponatremia via the ectopic production of ADH. Syndrome of inappropriate secretion of antidiuretic hormone (SIADH) is an important and common electrolyte disorder in tumour patients and one that has been reported in association with small cell lung cancers (SCLC). Schwartz et al. presented the first clinical case of a patient with ectopic SIADH in 1957, when he described two patients with lung cancer who developed hyponatremia associated with continued urinary sodium loss. We describe the patient with small cell lung cancer with SIADH. In chest radiogram as well as in computed tomography the pathological mass was described, later diagnosed as SCLC. The patient developed different neurological and psychical disorders that partially were improved after treatment.