Few epidemiological studies assessed the joint effects of chemical pollutants, aerobiological and meteorological factors on hospital admissions for asthma. However, there is much evidence that a relationship exists between the pollen count and both diesel exhaust and certain extreme weather conditions (such as thunderstorms). The aim of this study was to investigate the influence of environmental factors on asthma by determining the “discomfort days” for this disease. Daily counts of admissions for asthma among adults to the biggest regional hospital in Florence (A.O.C.: Azienda Ospedaliera di Careggi) were analysed for the period 1998-2001 (about 800 cases). Women showed higher prevalence than men to the ratio of 1.7F / 1M and the average age was 58. Hourly meteorological data was obtained from a station located in the city centre of Florence. Hourly air pollution data and the aerobiological factors were obtained from the Regional Office for Environmental Protection in Tuscany. We characterized a “discomfort day” on the basis of threshold values of meteorological factors (temperature, relative humidity, wind speed, atmospheric pressure, global radiation and rainfall), three biometeorological indices (Heat Index, Wind Chill Index and Equivalent Temperature Index) and chemical pollutants (SO2, NO2, CO, O3 and PM10). We defined a “discomfort day” as a day which caused an impact on human health, in relation to specific diseases and linked to environmental and meteorological parameters. Also the concentration and quality of aerobiological factors (pollens and fungal spores) were evaluated. Two-by-two contingency tables on the relationship between frequencies of admissions which occurred in so called “discomfort days” in comparison to those which were considered “comfort days” were calculated to assess which meteorological and environmental factors have a statistically significant impact on hospital admissions. A different monthly distribution of admissions for asthma was observed. Months with an average number of admissions significantly different in comparison with other months were February, May, July and August. Threshold values of biometeorological indices describing discomfort days which are characterized by sultry conditions were associated with admissions for asthma in both men and women (P < 0.05). Discomfort days with extended and high values of air temperature (i.e. temperature higher than 30°C for at least 6 hours, P=0.000039; maximum temperature > 34°C, P=0.00015) were significantly associated with admissions for asthma in women.