Bioclimatic indices, primarily devoted to the weather forecasts, in some cases are also used for the indoor thermal environment assessment with a consequent underestimation of both comfort and stress situations. The reason are: a) an empirical formulation; b) the need to collapse in few measurements and calculations the complexity of the physiological response of the human body which is affected by six parameters (air temperature, air velocity, relative humidity, mean radiant temperature, metabolic rate and clothing thermal insulation); c) they are usually validated under specific climatic conditions. On this point of view, the Universal Thermal Climate Index (UTCI), formulated in 2009, exhib-its a very innovative approach because: a) it is based on the well-known and most advanced Fiala’s thermoregulation model; b) it takes into account the whole of the parameters affecting the thermal sensation; c) it can be used in a wide range of climatic conditions (from the cold to the hot). On the base of so interesting peculiarities this paper deals with a comparison between the assessment carried out according to the methods suggested by ISO 7730 and UTCI index. Obtained results show that UTCI index can only provide a qualitative assessment of the thermal sensation in moderate environments.