Aims of research: to determine if a statistically significant result exists between the values of certain tumor markers in the serum and in according to the same tumor markers in the pleural effusion and to resolve if a correlation exists between the values of the same tumor markers. Examinees and actions: in this research 110 patients were examined, who came to the hospital with a diagnosis of pleural effusion in the period from November 4th 2013. to November 18th 2014. Patients were prospectively analysed, and of special interest was to determine four tumor markers (CA 125, CEA, NSE i CYFRA 21-1) in the pleural effusion which were then compaired to the same four markers in the serum. Programs Microsoft Excell 2010. and Statistica 12 were used for analyzing data. Results: when we compaired the four markers values in the serum with the values in the pleural effusion we got a statistically significant result between markers CA 125, NSE and CYFRA 21-1 at a level of statistical significance p<0.001, while CEA marker values showed no statistical significance (p>0.05). Also statistical tests showed that a statistically significant correlation (p<0.001) existed between the values in the serum and pleural effusion for all markers, that the correlation was positive and the highest in the CEA tumor marker. Conclusion: analyzing CA 125, NSE i CYFRA 21-1 we found a statistically significant result between the values of markers in the serum and pleural effusion and for that reason it is of unusual importance, to analyse these markers in pleural effusions. When reffering to the CEA tumor marker we did not find a statistically significant result between the values of the marker in the serum and pleural effusion thus further research is needed to determine its diagnostic relevance in the analysis of the pleural effusion in patients.