The relationship between weight loss, the systemic inflammatory response and quality of life in patients with inoperable non-small cell lung cancer (NSCLC) was studied. The extent of weight loss, the systemic inflammatory response (C-reactive protein) and quality of life (EORTC-QLQ-C30) was measured in 106 patients with inoperable NSCLC (stage III and IV). Approximately 40% had more than 5% weight loss and almost 80% had elevated circulating C-reactive protein concentrations (>10 mg/l). The functional scale scores of the EORTC-QLQ-C30 were poor (50 or less) and the fatigue symptom score was also poor (50 or more). When patients were grouped according to whether or not they had experienced more than 5% weight loss, Karnofsky performance status and global quality of life were lower (P<0.05) and symptom scores fatigue (P<0.05) and pain (P<0.01) were greater in the weight-losing group. When the weight-stable cancer patients were grouped according to whether or not they had evidence of a systemic inflammatory response, the symptom fatigue was higher in the inflammatory group (P<0.05). In the weight-stable cancer patients C-reactive protein concentration was correlated with fatigue r=0.31 (P<0.05). The results of the present study indicate that both weight loss and the systemic inflammatory response impact on different aspects of quality of life. In particular, fatigue is associated with the presence of a systemic inflammatory response independent of weight loss.