Legionella pneumophila is a bacterium, which can grow inwater pipe networks and climate systems. Contaminationoccurs by aspiration of infected water or aerosol inhalation.It is usually presented with fever, bradycardia, andchange in mental status, hyponatremia, elevation of liverenzymes and deterioration of renal function. The definitediagnosis is established by detection of the antigens andcultivating in the culture medium. Also, malign lung tumorscan encounter with the same clinical findings, so lungcancer should be remembered in differential diagnosis.The patient hospitalized for the Legionella pneumophiladue to the physical examination and laboratory findingsduring the first evaluation in the emergency department.However, further examinations pointed to the cancer. Weaimed to emphasize the probability of malignant tumorsin terms of hyponatremia, increase in the liver enzymes,and failure in the renal functions, which were usually experiencedin emergency unit. J Clin Exp Invest 2013; 4(3): 390-392Key words: Legionella pneumophila, pneumonia, lung malignanc