The likely roles of the eosinophil leukocyte in human disease are reviewed. The eosinophil is richly endowed with toxic cationic proteins and is able to mount a respiratory burst. Thus, eosinophils have the capability to damage various targets, and evidence exists that they do so during helminth infections and during the course of many hypersensitivity diseases. Here we discuss the role of the eosinophil in human onchocerciasis with particular attention to the Mazzotti reaction. We also discuss other diseases where eosinophil degranulation is seen, especially cutaneous diseases. Finally, the possible role(s) of the granule major basic protein in human pregnancy is noted.