Legionella pneumophila is a bacterium, which can grow in water pipe networks and climate systems. Contamination occurs by aspiration of infected water or aerosol inhalation. It is usually presented with fever, bradycardia, and change in mental status, hyponatremia, elevation of liver enzymes and deterioration of renal function. The definite diagnosis is established by detection of the antigens and cultivating in the culture medium. Also, malign lung tumors can encounter with the same clinical findings, so lung cancer should be remembered in differential diagnosis. The patient hospitalized for the Legionella pneumophila due to the physical examination and laboratory findings during the first evaluation in the emergency department. However, further examinations pointed to the cancer. We aimed to emphasize the probability of malignant tumors in terms of hyponatremia, increase in the liver enzymes, and failure in the renal functions, which were usually experienced in emergency unit.