The level of ash pollen grains (Fraxinus spp.) detected in the air in the city of Cordoba rarely surpasses the daily average of 8 g/m3 and is always detectable during the winter and the beginning of the spring. This fact, together with the knowledge of the presence of perennial symptoms in patients monosensitive to Olea in this area, allows us to suspect the possibility of cross-reactivity between both taxa. The skin tests carried out on a total of 1500 pollinotic patients with an extract of Fraxinus pollen offer us a global sensitization frequency of 59%. Furthermore, the Fx+ patients generally have a higher frequency of rural origin or they live in the most southerly district of the province. As for the age and sex of the patient, significant differences were found for the age groups between 5-10 years and over 25 but the frequency for both sexes was equal. As for the clinical features, differences in the family and personal antecedents of atopy were not observed, although rhinoconjunctival clinical antecedents seemed to prevail slightly over asthma antecedents. In relation to the duration of the symptoms, the patients presented a long evolution in their symptomatology. The great majority of the patients were polysensitive; only 8% were found to be monosensitive, with unimportant differences in their evolution after immunotherapeutic treatment. On the other hand, 92% of the Fx+ patients presented reactions to Olea pollen (but only 41% of the Fx- patients did), whereas 62% showed sensitivity to Cupressus, marking a strong contrast with the Fx-patients, who all tested negative for Cupressus pollen.(ABSTRACT TRUNCATED AT 250 WORDS)