Severe eosinophilia may be complicated by acute or chronic visceral damage. The underlying origin of the hypereosinophilia may be infectious, allergic, toxic, malignant or systemic (the secondary or reactive hypereosinophilic syndrome), but in a number of cases no cause can be found (the idiopathic hypereosinophilic syndrome). We describe 4 cases with hypereosinophilia and secondary visceral damage after residence in a tropical region. In three cases a helminthic infection was the obvious cause, the brain and the heart were the target organs. After treatment of the infection both the hypereosinophilia and the neurological and cardiac lesions disappeared. The fourth patient died of multi-organ disease. No definite trigger of the hypereosinophilia could be found. We discuss clinical findings, necessary investigations and therapeutic strategies.