Many studies have been carried out to assess the effect of climate on human health in an urban environment, in particular the effect of high temperatures in an urban area known by the name of “Urban Heat Island”. The aim of this study was to evaluate the modifications which occurred in thermohygrometric conditions during the 20th century in Florence as a consequence of the urban development (variation in the number of inhabitants, number of cars circulating, energy consumption for domestic and general use). This study also analysed the effects of these on the quality of life of the inhabitants. The data used was obtained from an urban meteorological station located in the city centre belonging to the Ximenes Observatory in Florence. This observatory has one of the oldest historical meteorological series in Italy, starting from 1813. In this study two meteorological series were used: a) a thermohygrometric series, with hourly data, for the period 1956-2000; b) a thermal series of daily maximum and minimum temperature, for the period 1889-2000. The first series was divided into three temporal groups (1956-1967; 1973-1986; 1987-2000) and then used to calculate biometeorological indices to evaluate the modifications in summer and winter discomfort conditions during the last 50 years. Three biometeorological indices were applied: i) Summer Simmer Index to describe discomfort conditions during summer; ii) Table of Scharlau to evaluate discomfort conditions during winter; iii) Thermohygrometric Index to describe climatic conditions during the whole year. The thermal series was utilized to analyse city temperature change in the 20th century. All these analyses were correlated with several urban parameters considered responsible for the worsening of the quality of life in an urban environment. In particular statistical analyses were performed by correlating thermal data and discomfort indices with the variation in the number of inhabitants, cars circulating and energy consumption for domestic and general use. Biometeorological indices applied to the historical meteorological series clearly showed an increase in the trend of acute discomfort for summer sultry conditions in the last group (1987-2000) in comparison with the other two groups. On the other hand, for winter, indices showed little variation among the three temporal groups. The twentieth century showed a strong decrease in days with low temperatures (i.e. Ta < 0°C and Ta < –5°C) and a notable increase of days with high temperatures (i.e. Ta > 34°C and Ta > 36°C) in the last 20 years. The winter minimum temperature, the summer maximum temperature and the yearly average temperature clearly showed a tendency to be rising for the period 1889-2000. Significant correlations were found between summer thermal and urban parameters, while winter minimum temperature was not significantly associated with all these parameters.