The relationship between circulating C-reactive protein concentrations and potential cytokine and receptor mediators (interleukin-6, leukaemia inhibitory factor (LIF), ciliary neurotrophic factor (CNTF), soluble IL-6 receptor, soluble gp130, soluble TNF receptor, interleukin-1 receptor antagonist and interleukin-8 (IL-8)) of this acute phase protein were examined in healthy subjects (n=11) and patients with non-small-cell lung cancer (n=50). Leukaemia inhibitory factor and CNTF were below detection limits in all controls and patients. C-reactive protein, interleukin-6, soluble gp130, soluble TNF receptor, interleukin-1 receptor antagonist and IL-8 concentrations were significantly elevated in cancer patients (P< or =0.001). Cancer patients with elevated C-reactive protein concentrations had greater concentrations of interleukin-6 (P<0.01) and interleukin-1 receptor antagonist (P<0.05). On regression analysis only interleukin-6 was independently associated with C-reactive protein (r=0.616, P<0.001). Interleukin-6 is an important independent mediator of elevated C-reactive protein concentrations in patients with non-small-cell lung cancer.