Although eosinophils produce cysteinyl leukotrienes (CysLTs) in large quantities, information on the relationship between CysLTs and eosinophilic pneumonia (EP) is lacking. Inflammatory mediator concentrations in urine were quantified to clarify the relationship between CysLT concentrations and EP severity. Leukotriene (LT)E(4), eosinophil-derived neurotoxin (EDN), 9alpha,11beta-prostaglandin F2 and LTB(4) glucuronide concentrations were quantified in the urine of: EP patients during acute exacerbation and clinical remission; asthmatic patients during acute exacerbation and under stable conditions; and healthy control subjects. The urinary LTE(4) and EDN concentrations of EP patients during acute exacerbation were significantly higher than those of asthmatic patients and healthy subjects, and decreased immediately during clinical remission. The urinary LTE(4) concentration was associated with the urinary EDN concentration of EP patients during acute exacerbation. The urinary LTE(4) concentration significantly correlated with the diffusing capacity of the lung for carbon monoxide in EP patients during acute exacerbation. The increased urinary concentrations of leukotriene and eosinophil-derived neurotoxin were associated with acute exacerbation in eosinophilic pneumonia patients. The increased leukotriene concentration significantly correlated with diffusing capacity of the lung for carbon monoxide, suggesting that the monitoring of leukotriene concentration may aid in the management of eosinophilic pneumonia patients.