Concentration of zinc in blood serum and effusion was determined in 104 patients with the pleural effusion of different etiology. The importance of zinc concentration in serum and effusion was analyzed, as well as their relation regarding the differential diagnosis of pleural effusion. It was established that the isolated zinc concentrations in serum and pleural effusion could not be used separately either in differing transudates from exudates or in the diagnosis of the pleural diseases. The average value of zinc in the pleural effusion in relation to the serum value in patients with tuberculosis effusion was 1.37, higher than 1 in all patients and was significantly different from the average value of the ratio 0,74 in patients with nonspecific and malignant pleural effusions. The relation of zinc concentration in the effusion and serum higher than 1.0 reliably indicated the presence of tuberculous pleurisy.