Serum levels of interleukin-6 (IL-6) were evaluated in a group of advanced non-small cell lung cancer (NSCLC) patients using an enzyme-linked immunosorbent assay. The data were related to clinical status and to cisplatin-based chemotherapy response. The mean IL-6 concentrations were higher than the controls (p<0.0001); patients with metastatic tumor had higher levels than those with undisseminated disease (p<0.006). Tumor progression was associated with an increase of IL-6 levels. Patients who responded to chemotherapy had lower serum IL-6 levels compared with unresponsive patients (p<0.0001). These data suggest that NSCLC patients with high levels of IL-6 have a worse clinical outcome and may manifest resistance to cisplatin chemotherapy.