Summary. Asthmatic patients receiving antileukotrienes may develop hypereosinophilia as a part of a Churg-Strauss syndrome. It is unclear if this effect is directly related to the administration of antileukotrienes or a consequence of the corticosteroid-sparing effect of antileukotrienes unmasking a Churg-Strauss syndrome. We present a case of hypereosinophilia related to zafirlukast therapy. The patient did not fulfil criteria for Churg-
Strauss syndrome. Symptoms and laboratory findings improved after zafirlukast was removed.

Key words: Antileukotriene, zafirlukast, hypereosinophilia, adverse reaction.