Dispersion of pollen from local ragweed sources was studied to determine the effect of contributions from such sources upon the pollen concentrations originating in more distant areas. Since ragweed pollen is produced throughout a large region, concentrations measured at any given location represent contributions from many sources at various distances along the past trajectory of the air sampled. A local source may produce concentrations several orders of magnitude above this background in a small downwind region. These concentrations decrease with distance and at some point become insignificant in comparison to background concentrations. Five experimental plots of common ragweed (Ambrosia artemisiifolia) were grown at Brookhaven National Laboratory over a four-year period. Pollen concentrations were measured by arrays of samplers located in concentric rings at several distances from each source. These concentrations were studied in relation to background pollen. The maximum downwind concentration is related to source size. Distances necessary for concentrations to reach specified fractions of background and the areas covered by concentrations greater than specified multiples of background are related to source size, surrounding vegetation, and meteorological conditions. These data should provide guidance to allergists and public health officials in evaluating the importance of local sources.