Several cases of eosinophilic conditions including Churg-Strauss syndrome (CSS) have recently been reported in asthmatic patients being treated with antileukotriene receptor antagonists. One patient with CSS who experienced a clinical relapse after treatment with montelukast and two asthmatic patients who developed CSS while receiving montelukast treatment are described. In one case reduction in the dose of oral steroid preceded the onset of CSS. To our knowledge, no case of CSS relapse has previously been reported in association with leukotriene antagonists.