Trees are considered producers of allergenic pollen. The aims of this work were to characterize the aerobiology of the Platanus, Acer, Salix, Quercus, Betula and Populus pollen, linking it with monthly emergency hospital admissions and to identify the different reactivity levels in sensitized patients. This information would be of great importance to evaluate the convenience of changing the inventory of pollen producer trees related to the risk of allergenic reactions. The study was conducted in Porto, Portugal, from 2005 to 2007. Airborne pollen was sampled using a Hirst-type volumetric trap. The antigenic and allergenic properties of Acer negundo, Betula pendula, Platanus occidentalis, Populus hybrida, Quercus robur and Salix babylonica pollen, collected in public gardens or sidewalks, were investigated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunological techniques using polysensitized-patient sera. Monthly hospital admissions of asthma or dyspnea related with respiratory diseases were obtained from the Emergency Room database of Hospital Geral de Santo António. Tree pollen and hospital admissions were positively correlated. Tree pollen peaked in March which coincides with the hospital admissions maximum. The highest binding affinity was observed with A. negundo, S. babylonica and P. occidentalis pollen extracts and the lowest with P. hybrida. Consistently, Acer and Platanus maximum airborne pollen concentrations were observed during March attaining levels considered moderate to high risk for allergenic reactions. Prominent bands with approximately 71, 35, 31, 22, 19, 16, 14, 13 and 11 kDa were revealed. A 52 kDa band was shared by all analyzed sera. High levels of airborne pollen and emergency hospital admissions were related. High binding affinity of specific IgE to pollen extracts of the most abundant tree pollen present in the atmosphere was observed. Patient sera revealed multiple similar allergenic bands shared by the different extracts. This multidisciplinary approach is useful in day-to-day medical practice to help in diagnostic, therapeutic and allergy alerting system adjusting.