Thirty eight patients from two geographical areas of Spain, with great differences in Olea europaea pollen counts were studied to investigate their in vivo and in vitro immune response to this pollen as a consequence of their different environmental allergen exposure. They were distributed in two groups (13 from Madrid, and 25 from Jaén). Skin sensitivity was assessed by a prick-test dose-response bioassay using serial dilutions of a biologically standardized allergen extract of O. europaea. Serological immune response was evaluated measuring specific antibody levels (IgE, IgG, IgG1 and IgG4). The patients from Jaén, who have a higher exposure to olive pollen, had higher levels of specific antibodies but significantly smaller wheal sizes than a similar patient population form the Madrid area, where olive pollen is not so copious. There is a great discrepancy between the results of skin prick tests (low cutaneous reactivity associated with high allergenic environmental load) and the levels of specific IgE to the olive pollen. While the level of specific antibodies increases with the allergenic load, the capacity to release mediators seems to be decreased, at least in the skin. Further studies are needed to evaluate if these findings also occur in other target organs with appropriate challenge tests (conjunctival, nasal and bronchial). This pattern should be studied with other allergens in large patient populations.