BACKGROUND: Eosinophilic pleural effusion constitutes a captivating and salient subject for exploration. In order to find out the reasons behind the formation of eosinophilic pleural effusion, we chose this study, hoping to gradually unravel its mysterious veil. METHODS: The absolute value and percentage of eosinophils in healthy people (normal control group), the peripheral blood of patients, and the pleural effusion of patients were compared to analyze their characteristics. RESULTS: In patients with eosinophilic pleural effusion, the absolute level of eosinophils was significantly greater than that in healthy people (normal control group) (p <
0.0001) and the percentage of eosinophils was significantly greater than that in healthy people (p <
0.0001) as well, even within the biological reference interval. CONCLUSIONS: Peripheral blood eosinophil counts were higher in patients with eosinophilic pleural effusion than in normal controls, even within the biological reference interval. Moreover, the overall absolute level of eosinophilic cells in the pleural effusion fluid was consistent with that in the peripheral blood. Combining these characteristics, the mechanism of eosinophilic pleural effusion formation was deduced.