Pneumonia is the second most common cause of pleural effusion. The presence of pleural fluid makes the prognosis in patients with pneumonia worse and causes a higher mortality rate. This is why it is very important to undertake suitable treatment as soon as possible. Most parapneumonic pleural effusions resolve with antibiotic alone, but sometimes more invasive treatment is necessary. Therapeutic decisions are based on different criteria. For many years determining biochemical fluid characteristics has been used for this purpose. Presentlythe usefullness of new parameters is being evaluated. Their application in diagnostics was possible thanks to better knowledge of mechanisms that participate in the development of parapneumonic pleural effusion.