Alternaria alternata (AA) is an important mould in respiratory allergic diseases. The objective is to determine the prevalence of AA sensitization in respiratory allergic patients and to examine the annual variation of Alternaria spores. 824 patients between 5 and 65 years old with allergic rhinitis and /or asthma were enrolled. Prick tests were performed with two different standardized AA extracts with quantification of Alt a 1. Alternaria spores concentrations were provided. 151 patients (18.3%) were sensitized to AA. The patients sensitized to AA were affected more frequently by asthma when compared to the patients not sensitized to AA (chi-square test = 7.34; p = 0.003). The prevalence of AA sensitization in paediatric population was statistically significantly higher than in patients older than 13. Atmospheric levels of Alternaria spores showed two periods of maximum concentration: July/August and October/November. The sensitization prevalence of AA in patients with respiratory allergy is meaningful, fundamentally in paediatric patients and/or allergic asthma patients. There is a significance spore concentration of Alternaria in the studied area, with a seasonal behaviour.