Many asthmatics are sensitive to rye-grass pollen, but pollen grains are too large to penetrate the lower airways. Our aim was to investigate the mechanism by which rye-grass pollen causes asthma. A major allergen of rye-grass pollen, Lol pIX, is located in intracellular starch granules within pollen grains. In-vitro tests showed that pollen grains are ruptured in rainwater by osmotic shock, each grain releasing about 700 starch granules into the environment. These granules are small enough to enter the airways (<3 μm in diameter). The starch granules were present in atmospheric samples taken during the pollen season, and showed a 50-fold increase in atmospheric concentration on days following rainfall. Isolated granules elicited IgE-mediated responses in asthmatic patients, and 4 patients with rainfall-associated asthma who underwent an inhalation challenge test had striking bronchial constriction after exposure to starch granules. Starch granules released from rye-grass pollen seem to be capable of causing asthma.