The presence of vascular endothelial growth factor (VEGF) was examined by enzyme immunoassay in 60 cytology-documented malignant pleural effusions associated with primary lung cancer and 51 other benign and malignant pleural effusions. Exudative pleural effusions contained significantly higher amounts of VEGF than transudative pleural effusions. Among exudative pleural effusions, levels of VEGF in malignant pleural effusions associated with lung cancer were significantly higher than those of benign exudative pleural effusions. There was no significant difference in pleural VEGF in patients with different histological types or clinical stages of lung cancer. Serial measurement of pleural VEGF levels was performed in six lung cancer patients treated with intrapleural instillation of recombinant interferon gamma, and reduction of pleural effusion was associated with decreasing pleural VEGF levels. These findings suggest that VEGF has a role in the accumulation of exudative pleural effusions, especially that of malignant pleural effusion associated with lung cancer.