The subject of this paper is the urban heat island of Cracow and its development during the period 1951–1980. Urban heat islands result from the absorption and storage of solar energy by the urban artificial substratum, and also from the heat released into the atmosphere from industrial and communal processes. The intensity and duration of the urban heat island depends on the density and character of the buildings. The relationship between the region's urban development and the size of its heat island was used to determine the boundaries of the heat island during the period 1951–1980 and its expansion toward external regions. The investigations showed that the extent of the heat island increased with the town's development.