Giant Ragweed (Ambrosia trifida) pollen is a cause of autumn herbaceous pollinosis in Japan. We describe the results of a study on airborne pollen dispersal for this species by mobile air samplers. The study site selected is an area on the banks of the Arakawa River in Saitama City. Mobile air samplers were positioned, in three well-spaced sites, within the catchment area of windborne Ragweed pollen. Collected pollen samples were counted using an optical microscope. The results show that the number of pollen grains per sample is less in the samples collected further away from the Ragweed source. However, it was observed that the quantities of Giant Ragweed pollen grains in the atmosphere, were considerable enough to cause significant allergenic reactions in local pedestrians, and residents living within the study area. It was also noted that the rate of pollen dispersal for Giant Ragweed is significantly affected by wind speed and direction, local to the pollen source. We consider that, for effective control of autumn herbaceous pollinosis, further data from airborne autumn pollen counts are needed, as well as clinical analyses of the allergen contents released from Ragweed pollen.