Plasminogen activator inhibitor-1 (PAI-1), the major circulating inhibitor of urokinase [urokinase-type plasminogen activator (uPA)], has been linked to the pathogenesis of lung cancer. PAI-1 belongs to the serpin family of inhibitors and inhibits both free urokinase (uPA) and receptor-bound urokinase (uPA receptor). Although PAI-1 has been related to a poor prognosis in lung carcinoma, mechanisms that regulate its expression in human lung cancer cells are not well understood. We used cultured human small cell and non-small cell lung carcinoma cell lines as model systems to elucidate the regulatory mechanisms that control expression of PAI-1. Levels of PAI-1 protein were significantly increased in selected lung carcinoma cells compared with those in normal small-airway epithelial cells. Corresponding steady-state levels of PAI-1 mRNA were similarly increased in these cells. The half-life of PAI-1 mRNA was prolonged in these lung carcinoma cell lines after transcriptional or translational blockade. We identified a 60-kDa protein that binds the 3'-untranslated region of PAI-1, and complex formation of this binding protein with PAI-1 mRNA reciprocally correlates with mRNA stability. The findings demonstrate that expression of PAI-1 is regulated at the posttranscriptional level in small cell- and non-small cell-derived human lung carcinoma cell lines. Altered regulation of PAI-1 at the posttranscriptional level may contribute to relative overexpression by malignant lung epithelial cells. A newly identified regulatory protein that binds to the 3'-untranslated region of PAI-1 mRNA appears to be involved in the posttranscriptional regulation of PAI-1 gene expression by human lung carcinoma cells in vitro.