['Weight loss is frequently seen in advanced cancer. Bioelectrical impedance spectroscopy (BIS) is a convenient method for estimating body composition. We examined in a prospective, comparative study if BIS could accurately estimate fat-free mass (FFM) in cancer patients compared to dual-energy X-ray absorptiometry (DXA).', 'The study was based on 132 consecutive incurable cancer patients with solid tumours in a University hospital outpatient clinic. Comparison of FFM from DXA and BIS with standard and revised equations. Bland-Altman plots, t-tests and linear regression analysis were used to evaluate agreement and differences between methods.', 'BIS significantly underestimated mean FFM with 7.6+/-4.7 kg compared to DXA (P<0.001). Bias was significantly correlated to % weight loss (r=0.32), systemic inflammation as measured by C-reactive protein (r=0.29), malnutrition as assessed by low insulin-like growth factor-1 (r=-0.23) and inversely to the per cent body fat estimated by DXA (P=-0.61) and body mass index (BMI; r=-0.30). Revised BIS equations taking BMI into account reduced bias significantly but still with great individual variation.'] Does bioelectric impedance spectroscopy underestimate fat-free mass compared to dual energy X-ray absorptiometry in incurable cancer patients?