Background: Although seasonal patterns of tree pollination have been reported, it is unknown if aerobiologic data correlate with patterns of in vivo sensitization. Objective: To evaluate the relationship between regional tree pollen exposure and patterns of in vivo percutaneous reactivity to specific tree pollen extracts in a local patient population with seasonal allergic rhinitis. Methods: Patients with spring seasonal allergic rhinitis and percutaneous sensitivity to 1 or more regional tree pollens were studied. Tree pollen counts were collected at the same urban site from 1997 to 2002 and at a suburban site in 2002. Patients underwent skin prick testing with commercial extracts of 15 indigenous tree species. Serum specific IgE measurements were assayed in a subset of sensitized patients. Results: Of 127 patients who reported symptoms consistent with seasonal allergic rhinitis during the spring pollen season, 93 qualified based on demonstration of at least 1 positive skin prick test result. Mean 5-year pollen counts (1997-2001) and 2002 urban counts were highly correlated (Spearman r = 0.95, P <.001), indicating that year-to-year pollen counts were consistent. No significant correlation was found between mean seasonal pollen counts (urban site, 1997-2001) and frequencies of skin prick test reactivity to specific tree pollen allergens (Spearman r = -0.03, P =.93). No significant relationship was found between 5-year mean tree pollen counts and positive serum specific IgE tests for specific tree pollens (Spearman r = -0.42, P =.30). Eight of 15 species elicited percutaneous reactions in more than 50% of patients (ie, satisfying definition of a major in vivo allergen). However, 6 of the 8 major tree allergens each represented 5% or less of 5-year mean total tree pollen counts. Conclusion: No correlation was found between overall frequencies of in vivo sensitization to tree pollen allergens in a local population and regional pollen exposure data.