The eosinophil has a potent armory of proinflammatory mediators with considerable potential to initiate and sustain an inflammatory response. These include cytotoxic granule proteins, cytokines, chemokines, and lipid mediators. Eosinophils are considered important in the immune response to infection with helminthic parasitic worms. Incrementally increasing evidence supports a critical role for their proinflammatory activities in diverse human conditions, most notably in allergic diseases such as asthma. In these conditions severe tissue damage is a consequence of an inappropriate accumulation of eosinophils and the subsequent release of their highly toxic granule proteins. In addition, release of granule-associated products such as chemokines and cytokines at the sites of inflammation is likely to have significant paracrine and autocrine relevance. This review will update recent developments in understanding the role that eosinophil granule proteins play in human disease, particularly those of the respiratory tract.