Core body temperature (CBT), arterial blood pressure, pulse, respiration and pain are basic vital signs and indicators of an individual’s health status. Changes in physiological functions are reflected in the values of an individual’s basic vital signs. Deviations from the normal values of vital signs indicate the disruption of homeostasis. Fever results from a cytokine mediated reaction that results in the generation of acute phase reactants and controlled elevation of core body temperature. Fever is an adaptive response to a variety of infectious, inflammatory, foreign stimuli and surgery. The incidence of fever ranges between 28% and 75% in critically ill patients, and fever has an infection and non-infectious causes. About 50% of fevers in ICU patients are due to infectious causes. On the other hand, between 40-50 % of patients develop fever after surgery depending on type of surgery but only a small percentage turn out to be due to infection. Fever of unknown origin remain one of the most common and difficult diagnostic problems faced daily by clinicians. In addition, pattern of temperature changes (continuous fever, intermittent fever, quotidian fever, tertian fever, quartan fever, remittent fever, pel-ebstein fever and neutropenic fever) may occasionally hint at the diagnosis. CBT increase to be followed by increase of oxygen consumption and energy expenditure. These increases in the metabolic rate and serum levels of stress hormones are suggested to subsequently change in haemodynamic parameters. Non-pharmacological and pharmacological methods are used to reduce CBT in febrile patients, but little researchs related to the effects of these methods on guests have been performed. Among critically ill patients, the effect of antipyretics on survival in patients with sepsis is unclear. Although the use of antipyretics to treat fever among patients with presumed severe sepsis may increase the risk of mortality in this setting. On the basis of these data, there is a plausible biological rationale that the presence of fever has different implications in patients with infection compared with those without infection. It is important for physicians/nurses to appreciate the causes of fever in medical-surgical patients and physiological effects of fever related on causes on haemodynamic parameters which can cause complications in these settings. Furthermore accurate and careful patient’s assessment and monitoring during febrile episodes can be helpful to determine the process of fever treatment choice and effects of thismethods on haemodynamic parameters and complication of treatment methods such as morbidity and mortality.