Rationale: Low body mass index (BMI) is a marker of poor prognosis in chronic obstructive pulmonary disease (COPD). In the general population the harmful effect of low BMI is due to the deleterious effects of a low fat free mass index (FFMI, fat free mass/weight(2)). Objectives: We explored distribution of low FFMI and its association with prognosis in a population based cohort of COPD patients. Methods: We used data on 1,898 COPD patients identified in a population-based epidemiological study in Copenhagen. Fat free mass was measured using bioelectrical impedance analysis. Patients were followed up for a mean of 7 years and the association between BMI and FFMI and mortality was examined taking age, gender, smoking and lung function into account. Main results: The mean FFMI was 16.0 kg/m(2) for women and 18.7 kg/m(2) for men. Among subjects with normal BMI, 26.1 % had an FFMI lower than the lowest FFMI 10(th) percentile of the general population. BMI and FFMI were significant predictors of mortality, independent of relevant covariates. Being in the lowest FFMI 10(th) percentile of the general population was associated with a hazard ratio of 1.5 (95% confidence interval 1.2-1.8) for overall mortality and 2.4 (1.4-4.0) for COPD-related mortality. FFMI was also a predictor of overall mortality when analyses were restricted to subjects with normal BMI. Conclusions: FFMI provides information in addition to BMI and assessment of fat free mass should be considered in the routine assessment of COPD.