Concentration of tumor-associated trypsin inhibitor (TATI) in pleural effusions. We measured the concentration of tumor-associated trypsin inhibitor (TATI) in plasma and pleural fluid of 84 patients with pleural effusions of various causes. We observed elevated (greater than 30 micrograms/L) TATI levels in pleural fluid in 45 percent of patients with pleural effusion associated with malignant disease and in 15 percent of patients with benign disease. Similar results were obtained for TATI in plasma. The concentration of TATI in pleural fluid closely parallelled that in plasma. In patients with renal insufficiency and in patients with biliary obstruction, the TATI levels were elevated both in plasma and pleural fluid. A positive correlation was seen between the concentration of TATI and the activity of alkaline phosphatase in plasma. The results show that simultaneous determination of TATI in plasma and pleural fluid improves the diagnosis of cancer only marginally. Our results also support the hypothesis that elevated TATI levels may reflect an acute phase reaction caused by inflammatory disease or tissue destruction associated with cancer not only in inflammatory conditions, but also in malignant disease where the tumor itself is not producing TATI. 