Measured data in some tropical Brazilian cities suggest that temperatures are increasing as the cities grow and the buildings become taller. The climate change in these urban areas noticeably increases human discomfort and air-conditioning usage in buildings. This paper deals with a case study carried out on the city of Belo Horizonte, Brazil, to quantify the effects of the urban climate change in energy consumption for cooling our typical office plant. It summarizes thermal-energetic performance simulations of a 10-storey office building, which were made using two external city climate conditions - built-up outskirts and downtown. The results were compared with the reference case, which was performed using the city average climate data for the same period. Both typical summer and winter conditions were simulated, but the results were significant only for our warm and dry winter (the rainy summer in Belo Horizonte makes difficult the development of the urban heat island phenomenon). This study indicates a monthly energy consumption increase of about 6% between the built-up outskirts and the reference case conditions, which reaches to 17% when the comparison was between the downtown and the reference case conditions. Therefore, not only policies to improve the buildings thermal-energetic performance have to be adopted. Also urban planning and design have to develop procedures and legislation to control urban land use and construction for saving energy and improving thermal comfort in the city through the reduction of the urban heat island effect.