Various kinds of cells and their mediators are thought to be involved in the pathogenesis of bronchial asthma. However, changes in each mediator or relationship among mediators during an asthmatic attack have not been well documented. In this study, to clarify whether eosinophil protein X (EPX) is a marker which is distinct from leukotriene E4 (LTE4), or 11-dehydrothromboxane B2 (11DTXB2), we measured the urinary excretion of EPX, LTE4, and 11DTXB2 in 14 asthmatics who were admitted to the hospital with either an acute asthmatic attack or status asthmaticus. These patients included eight atopic and six non-atopic types of bronchial asthma, with a median age of 34.0 years. Urinary excretion of EPX was significantly high on admission with the asthmatic attack, and returned to control levels 175 [122 –384] μg/day when the patients were in the improved state (1036–317 μg/day, P < 0.01). Similar findings were observed in LTE4 (155–59 ng/day, P < 0.01) and 11DTXB2 (991–442 ng/day, P < 0.01). No significant differences in values were observed between atopic and non-atopic types of asthma in all three substances. When the individual data during the attack state were analysed, a significant correlation was observed between changes (%) in urinary EPX and those in urinary LTE4, but no such relationship was observed between changes (%) in urinary EPX and those in urinary 11DTXB2. These results suggest that measuring urinary EPX levels may be a useful marker for the understanding and management of the disease.