Background: Work physiology besides providing the theoretical background for occupational medicine, is also concerned with performing practical research. The research is aimed at setting numerous physiological criteria for the assessment of workload and its maximum admissible values. Energy expenditure is commonly used as an index of work severity (or occupational work load). It is determined to assess the systemic load associated with performing various activities during occupational work, sports, rehabilitation treatment, etc. Human energy expenditure can be measured via direct or indirect calorimetric or tabular methods. The existing methods are encumbered with various disadvantages and the search for new, possibly simple but reliable methods, continues. Material and Methods: Based on the literature data, results of our own research, as well as the laws of heat exchange and fluid flow, a new model of temperature control system has been developed, which comprises some elements of the human circulatory system. This model has made it possible to develop our own, simple method for the assessment of energy expenditure, in Watts (W), and energy expenditure per human body area (or energy expenditure density), in Watts per square meter (W/m²). Th e new method involves measurements of inner body temperature, body mass and height to assess energy expenditure. Results: The results of energy expenditure assessments obtained using our new method do not diff er signifi cantly from those obtained in the same subjects with the indirect calorimetric method. Conclusions: Our new method may be considered as an alternative for existing methods of energy expenditure assessment. Med Pr 2008;59(3):215-222