Previous analyses of pleural effusions of different origin revealed high levels of matrix metalloproteinase-9 proenzyme (proMMP-9), predominantly in parainfectious exudates. To confirm the etiological association with inflammation and to explore time course of release of proMMP-9 under conditions of a local inflammatory reaction, the authors estimated proMMP-9 in pleural fluids of patients undergoing therapeutic talc pleurodesis (n = 20) and explorative thoracoscopy (n = 4), before, and at different time intervals after the intervention. The authors also compared proMMP-9 values with proinflammatory markers, such as interleukin-6 (IL-6), C-reactive protein (CRP), and leukocyte count. As revealed through scanning substrate electrophoresis, proMMP-9 levels increased within the first hours a!nd culminated 16 to 48 hours after intervention, amounting to 1.5- to 102 -fold of initial concentration (mean = 14.8 -fold). MMP-9 proenzyme values significantly correlated with the levels of proinflammatory cytokine IL-6 (r =.79, n = 47, p <.01) and with neutrophil counts (r =.75, n = 45, p <.01). In conclusion, the results present proMMP-9 as a novel early indicator of acute host-response in pleural space that complements traditional proinflammatory markers and might be useful in monitoring pleural inflammatory processes.