The Churg-Strauss syndrome (CSS) is a distinct form of vasculitis that is notable for its eosinophilia and frequent associations with asthma and sinusitis. Because there has been an increasing recognition that CSS can develop in patients with asthma and that CSS might be associated with specific asthma treatments, the National Heart, Lung, and Blood Institute, the National Institute of Allergy and Infectious Diseases, the Office of Rare Diseases, National Institutes of Health, and the US Food and Drug Administration jointly sponsored a workshop to consider interrelationships among CSS, asthma, and asthma therapeutics and to assess what is known about underlying mechanisms of CSS. Issues related to the criteria for defining and diagnosing CSS were reviewed, including the contemporary understanding that diagnostic biopsies need only reveal eosinophilic perivascular infiltrates and that asthma need not be present when CSS develops. From published reports and reports to the US Food and Drug Administration, treatment of patients with asthma with any of 3 cysteinyl leukotriene receptor antagonists, a 5-lipoxygenase inhibitor, and inhaled corticosteroids has been associated with CSS development. It is unknown whether these agents were eliciting CSS. A variety of physiologic and study design issues might lead to the reported associations of these drugs with CSS. Because many asthma patients receiving these therapies were able to diminish their systemic corticosteroid therapy, it is possible that incipient CSS was unmasked by lessened steroid use. The underlying pathophysiologic mechanisms of CSS, however, are unknown, and there is no means of identifying which patients with asthma might be at risk for CSS. Accordingly, investigations with the goals of defining the underlying pathophysiologic processes of CSS and establishing the relationships of asthma and its therapies to CSS are needed.