Ragweed is an allergenic weed of public health concern in several European countries. In Italy ragweed occurs prevalently in north-north-eastern regions, where sensitization is increasing. Because of the small diameter of pollen grains, ragweed pollen is often involved in episodes of long-range transport, as already shown in central Italy. The objective of this study was to evaluate the extent of such transport by comparing pollen and meteorological data for two northern Italian cities (Parma and Mantova) with data from Pistoia and Florence in central Italy. In 2002 and 2004 peaks in ragweed pollen levels were detected in these four cities on the same day, and concentrations of the grains were above clinical thresholds. Weather-map analysis and computation of back-trajectories showed that air masses from eastern Europe might carry ragweed pollen to a wide area of central and northern Italy. These findings suggest that episodes of long-range transport of ragweed pollen could be clinically relevant, resulting in sensitization of a large number of people. The results might provide a basis for monitoring and forecasting periods of long-distance transport with the objective of reducing their effects on allergic patients.