Reduced food intake is a major cause of continuing weight loss in patients with cancer. Previous work has suggested that an ongoing inflammatory response may also contribute to weight loss and alter the nature of body tissue loss. To examine this, body cell mass was estimated using measurements of total body potassium (TBK) in 31 patients with gastrointestinal cancer and weight loss with or without an inflammatory response (C-reactive protein level above 5 mg/l). Albumin levels, total body water and 24-h urinary creatinine clearance were also measured. When measured TBK was expressed as a percentage of predicted normal values there was a significant reduction in TBK for patients with an inflammatory response compared with that of those without (P = 0.04). However, when a different prediction equation for TBK was used this difference was not significant (P = 0.29). Therefore, it remains uncertain whether an ongoing inflammatory response in patients with cancer and weight loss contributes to loss of body cell mass.