Pleural fluid analysis was performed in patients with idiopathic spontaneous pneumothorax. The objective of the study was to define the cell differentiation, and part of the cytokine profile, in relation to the duration of pneumothorax. In the 23 consecutive patients (19 men, mean age 34.2 years, 17 smokers), pleural fluid was obtained immediately after chest tube drainage (n = 6), or during thoracoscopy (n = 17). Cytospins were carried out, and supernatant analysis of the different cytokines was performed using sandwich ELISA. All concentrations were corrected for dilution. The duration of the pneumothorax was correlated with the rise in eosinophil percentage (r = 0.81, P < 0.00001) in pleural fluid. RANTES, platelet-activating factor (PAF), and monocyte chemotactic protein-1 (MCP-1) were detectable but no relationship with eosinophils or duration of the pneumothorax was found. Granulocyte-macrophage colony stimulating factor (GM-CSF) and interluekin-8 (IL-8) were not detectable. Interleukin-5 (IL-5) concentration correlated with the eosinophil concentration (r = 0.84, P = 0.037) and the eosinophil percentage (r = 0.68, P = 0.005) in the pleural fluid. Idiopathic spontaneous pneumothorax causes a time-related rise in the eosinophil percentage in the pleural space, which correlates with the level of IL-5.