Objectives: The phenomenon of urban heat island (UHI) is based on the occurrence of elevated air temperature in urban areas, in relation to the surrounding urban, rural and forest areas. The aim of the study was the assessment of the UHI in Łódź in 2014–2019. Łódź is a large city with an area of 293.3 km², located in the center of Poland. Material and Methods: The UHI was defined as the difference of at least 1.5°C in daily minimum temperature at the point representing the commutative conditions (lower temperature) and daily minimum temperature at the point located in the city center (higher temperature). Based on data from the Weather Research and Forecasting (WRF)/CALMET models and the above criterium, the occurrence of UHI cases was analyzed. Results: In summary the phenomenon of urban heat island in 2014–2019 was observed in every studied year, most often in 2018 with the highest UHI index almost 5°C, and increased over the years. The results proved that the highest UHI contrast was seen at night between 11 p.m. and 3 a.m. Conclusions: The study confirms that the UHI phenomenon in Łódź is persistent and has expanded in spatial extent over the years due to urban growth. Int J Occup Med Environ Health. 2024;37(6):581–90