The regulation of acute phase protein production and the relationship of the acute phase protein response to tumour growth was examined in colorectal cancer patients (n = 9). Ibuprofen (1200 mg/d) was administered for 8-11 days. Following ibuprofen administration there were reductions in circulating concentrations of C-reactive protein (P = 0.01), interleukin-6 (P = 0.06), cortisol (P = 0.04) and also in the platelet count (P = 0.01). There was no significant change in albumin, insulin and carcinoembryonic antigen. These results indicate that ibuprofen administered over a prolonged period substantially reduces acute protein production via its effect on interleukin-6 and cortisol. It remains to be determined whether ibuprofen is useful in moderating tumour growth in colorectal cancer patients.