The main goal of this paper was to assess the intensity of heat stress in Lower Silesia, Poland, during selected weather events characterized by high air temperatures. The complex impact of weather on the thermal load of the human organism is presented using the Universal Thermal Climate Index (UTCI). The analysis was carried out for the 2015 and 2018 summer seasons and compared with the multiannual period of 1971–2018. It was based on meteorological data from the IMGW-PIB stations of Wrocław, Jelenia Góra and Śnieżka. In order to examine how heat conditions affect UTCI in different geographical regions, stations located at different altitudes and representing the lowlands, the lower mountain zone and the summit zone of the Sudetes Mountains were considered. The research showed that during the most extreme thermal events, UTCI values in the lowlands and the lower mountain zones can be among the highest heat stress classes. In the summit zone, the maximum UTCI values are usually classed as ‘no thermal stress’.