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)). We explored distribution of low FFMI and its association with prognosis in a population-based cohort of patients with COPD. We used data on 1,898 patients with COPD identified in a population-based epidemiologic study in Copenhagen. FFM was measured using bioelectrical impedance analysis. Patients were followed up for a mean of 7 yr and the association between BMI and FFMI and mortality was examined taking age, sex, smoking, and lung function into account. 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 10th percentile of the general population. BMI and FFMI were significant predictors of mortality, independent of relevant covariates. Being in the lowest 10th percentile of the general population for FFMI 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. FFMI provides information in addition to BMI and assessment of FFM should be considered in the routine assessment of COPD.