The concentration of pollen grains in the atmosphere over the metropolitan area of Monterrey, Nuevo Leon, Mexico, was analyzed throughout a year from March 2003–February 2004, focused on the genus Carya, Celtis, Cupressus, Fraxinus and Pinus owing to their interest as etiological pollinosis agents in diverse regions of the world. A 7-day Hirst type volumetric spore and pollen trap was located on a building roof of the city at 15 m from ground level for continuous sampling. The total quantity of pollen recorded for the study period was 21,083 grains/m³, corresponding to 49.75% of the taxa of interest. February and March were the months with higher pollen amounts in the air with 7,525 and 2,781 grains/m³, respectively, and amounted to 49% of total year through pollen. Fraxinus was the genus which contributed to the largest amount of pollen with 28% of total grains (5,935 grains/m³) followed by Cupressus with 13% (2,742 grains/m³). Celtis, Pinus and Carya contributed with 5.3%, 2.7%, and 0.6% of total pollen, respectively. These results indicate that Fraxinus and Cupressus are present in the area in suffi cient quantity to indicate likely involvement in the origin of allergic disorders in the human population.