The objective of the study was to evaluate the number of myeloid (DC1) and lymphoid (DC2) dendritic cells in malignant pleural effusion and peripheral blood of patients with lung cancer as well as to estimate changes in their presence during intrapleural interferon alpha treatment. The study comprised eight subjects including two patients treated with Roferon A. Isolated mononuclear cells were phenotyped with the following monoclonal antibodies: anti-BDCA-1 FITC and anti-BDCA-2 FITC, and analyzed using FACSCalibur flow cytometry. Very low percentage of DC2 and high BDCA-1/BDCA-2 ratio in malignant pleural effusion of patients not treated with IFN-alpha suggest intensive Th1 response probably responsible for inflammatory state maintenance and effusion constant development. The results of our study indicate that apart from the suggested antitumour activity, IFN-alpha therapy may result in unfavourable immunotolerance caused by increased DC2 activity.