In the present report, we serially measured the levels of interleukin-6 (IL-6) and some acute-phase proteins (APP) in 61 lung cancer patients undergoing radiotherapy in order to investigate the relationship between the response to the treatment and the changes in parameters of systemic inflammatory response. The patients were divided into two groups depending on the response to the treatment. The first group (referred to as responders) comprised 32 patients with stable disease, partial remission or total remission. Twenty-nine patients with progression of the disease were included to the second group (referred to as non-responders). Six patients died due to the lung cancer during the study. We showed a decrease in IL-6 serum level and C-reactive protein (CRP) level in responders but not in non-responders. However, the most interesting results were obtained after retrospective analysis of the data of six deceased patients. In these patients we observed an elevation of IL-6 and CRP before the patients' deaths. Following the changes in acute-phase response and interleukin-6 serum levels in lung cancer patients seems to be helpful in prognosis of the outcome of the disease. Based on our data, we conclude that an elevation in IL-6 and/or CRP level in patients with lung cancer may serve as an adverse prognostic factor.