Antileukotriene drugs are new therapeutic agents that have recently been approved for the treatment of asthma. Several cases of eosinophilic conditions including ChurgStrauss syndrome have been reported to be associated with zafirlukast, a cysteinyl leukotriene type 1 receptor antagonist. So far no other leukotriene modifier has been associated with the syndrome. The case history is presented of a man with allergic rhinitis and asthma who had received intermittent pulse therapy with oral corticosteroids. Pulmonary eosinophilia developed while he was receiving treatment with montelukast, a chemically distinct cysteinyl leukotriene type 1 receptor antagonist. After discontinuation of montelukast therapy and administration of systemic corticosteroids the patient's symptoms reversed rapidly and there was prompt resolution of the pulmonary infiltrates. We believe that cysteinyl leukotriene type 1 receptor antagonists are safe and eVective drugs for most patients with asthma but caution is needed for those with more severe disease who require systemic corticosteroids, especially if they show characteristics of the atypical allergic diathesis seen in the prodromal phase of Churg-Strauss syndrome. (Thorax 1999;54:558-560) Keywords: montelukast; side eVects; pulmonary eosinophiliaThe antileukotriene drugs are new therapeutic agents which have recently been approved and are now available for the treatment of asthma in several countries. These drugs include one enzyme inhibitor of 5-lipoxygenase, zileuton, and three chemically distinct cysteinyl leukotriene type 1 receptor antagonists, zafirlukast, pranlukast and montelukast. As the use of these drugs increases, adverse events occurring at low frequency or in populations not examined in clinical trials may become manifest.2 Several cases of eosinophilic conditions including Churg-Strauss syndrome have been reported in patients who have been treated with zafirlukast. [2][3][4] No other leukotriene modifier has yet been associated with the syndrome.In this case report we describe an asthmatic patient in whom pulmonary eosinophilia developed while receiving montelukast therapy. Case reportA 26 year old man with a three year history of asthma had received treatment with salbutamol, nedocromil, theophylline, and beclomethasone. There was a five year history of allergic rhinitis with positive skin tests for house dust mite and cat dander. Because asthma symptoms were not well controlled, treatment was started with fluticasone 1000 µg daily and salmeterol twice a day. Allergen immunotherapy was also prescribed. Nonetheless, short courses of oral prednisone 40 mg or deflazacort 60 mg daily were required on several occasions to control asthma exacerbations. A decision was made to initiate treatment with montelukast at a dose of 10 mg daily in the evening. Treatment with fluticasone was continued.After approximately four months of treatment with montelukast the patient developed headache, malaise, myalgia, nasal congestion, and fever up to 39ºC. Oral cefuroxime and parac...