The aim of this study was to explore the presence of interleukin (IL)-16 in pleural effusions, the correlation between IL-16 levels and cytological parameters, as well as the chemoattractant activity of IL-16 on CD4+ T-lymphocytes. Total nucleated cell and differential counts, and IL-16 concentrations in the pleural effusion from 32 patients with tuberculous pleurisy and 30 patients with lung cancer were determined. Threecolour flow cytometry was performed to determine T-lymphocyte subsets in cell pellets of pleural effusion. The chemoattractant activity of IL-16 for CD4+ T-lymphocytes was also analysed. The levels of IL-16 were significantly higher in tuberculous than in malignant effusions. However, IL-16 levels could not be used for diagnostic purposes due to significant overlap between the two groups. Positive correlations were found between the IL-16 levels and CD4+ Tcells, and pleural fluid was chemotactic for CD4+ T-cells in vitro. Intrapleural administration of IL-16 to patients produced a marked progressive influx of CD4+ T-cells into the pleural space. Compared with malignant pleural effusion, interleukin-16 appeared to be increased in tuberculous pleural effusion. Interleukin-16 levels were positively related to the numbers of CD4+ T-cells, and interleukin-16 could directly induce CD4+ T-cell infiltration into the pleuralspace.