Progastrin-releasing peptide (ProGRP) is a specific and actively secreted product from small-cell lung carcinoma (SCLC) cells. Recently, an enzyme-linked immunosorbent assay, which uses monoclonal and polyclonal antibodies to recombinant proGRP(31-98), a common region of ProGRP, was established. We measured concentrations of ProGRP (31-98), neuron-specific enolase (NSE) and carcinoembryonic antigen (CEA) in carcinomatous and infectious pleural effusions. Significantly increased ProGRP(31-98) and NSE values were found in carcinomatous pleurisy due to SCLC compared to the other carcinomatous pleurisy and infectious pleurisy. CEA values were significantly increased in carcinomatous pleural effusions compared with those in infectious effusions. The ProGRP(31-98) values were not correlated to NSE values in carcinomatous pleurisy due to SCLC. The determination of ProGRP(31-98) in pleural effusions will be helpful for diagnosing carcinomatous pleurisy due to SCLC.