There is recent evidence that the inflammatory response may be important in the disproportionate loss of body cell mass in cancer patients. To examine this further, 18 male patients with lung or gastrointestinal cancer were studied over a 12-week period. In addition to weight, anthropometry, C-reactive protein (marker of the inflammatory response), albumin, and total body potassium were measured at baseline and 12 weeks. When those patients who lost total body potassium were compared with those who had not, there was a significant increase in the baseline and 12-week C-reactive protein concentrations (p < 0.05). The reduction in total body potassium was also associated with a reduction in triceps skinfold thickness (p < 0.05). There were significant correlations between the mean C-reactive protein concentration and the relative (r = -0.846, p < 0.001) and absolute (r = -0.806, p < 0.001) change in total body potassium over the follow-up period. This study demonstrates the association of a chronic inflammatory response with the rate of loss of body cell mass observed in cancer patients.