Leukotrienes are potent pro-inflammatory mediators that appear to contribute to pathophysiologic features of asthma. For example, cysteinyl leukotrienes contract airway smooth muscle, increase microvascular permeability, stimulate mucus secretion, decrease mucociliary clearance, and appear capable of recruiting eosinophils into the airways. Segmental antigen bronchoprovocation in patients with asthma increases LTC4 concentrations in bronchoalveolar lavage fluid, which correlates with an influx of eosinophils into the airways. LTB4, in comparison, selectively affects neutrophil functions. Intratracheal instillation of LTB4 produced a selective recruitment of neutrophils into the lung. These effects suggest that leukotrienes contribute significantly to the inflammatory components of asthma.