The relationship between urban growth and the formation of urban heat islands, i.e. climatic differences between the urban area and adjacent rural areas, is discussed by several authors and is assumed to be ubiquitous for various climatic regions. Curitiba (25.5ºS), located within a region of subtropical climate in elevation, boasts a population growth rate of approximately 2% a year. The purpose of this study is to evaluate the effect of the urban agglomeration on microclimate changes. From comprehensive long-term climate data monitoring carried out during 2011-2013 with a pair of weather stations in and outside the urban area, outdoor thermal comfort conditions are assessed by means of the UTCI index, which had been calibrated to local thermal preferences in a previous study. Comparisons between urban and rural conditions are shown as traditional Urban Heat Island (UHI) results as well as in terms of predicted comfort/discomfort levels. A projection to a more densified condition is performed by significantly reducing air speed at night. We discuss obtained results in predicted thermal comfort variations against current conditions.