We have measured the serum concentration of the acute phase reactant, C-reactive protein (CRP), in 20 patients with histologically proven small cell lung cancer undergoing their first pulse of induction cytotoxic chemotherapy. Baseline CRP concentrations were raised in 16 of 20 patients (median baseline CRP 18.5 mg l-1; normal range less than 10 mg l-1). CRP levels more than doubled in 11 of 20 patients during induction chemotherapy. This acute phase reaction was seen in seven of the 10 chemosensitive patients, but was not observed in any of the five non-responding patients. Five patients were non-evaluable for chemoresponse. These data indicate that there is a previously undescribed quantifiable acute phase response during chemotherapy for small cell lung cancer which has potential for predicting chemoresponse.