The association between low body mass index (BMI) and poor prognosis in patients with chronic obstructive pulmonary disease (COPD) is a common clinical observation. We prospectively examined whether BMI is an independent predictor of mortality in subjects with COPD from the Copenhagen City Heart Study. In total, 1,218 men and 914 women, aged 21 to 89 yr, with airway obstruction defined as an FEV(1)-to-FVC ratio of less than 0.7, were included in the analyses. Spirometric values, BMI, smoking habits, and respiratory symptoms were assessed at the time of study enrollment, and mortality from COPD and from all causes during 17 yr of follow-up was analyzed with multivariate Cox regression models. After adjustment for age, ventilatory function, and smoking habits, low BMI was predictive of a poor prognosis (i.e., higher mortality), with relative risks (RRs) in underweight subjects as compared with that in subjects of normal weight of 1.64 (95% confidence interval [CI]: 1.20 to 2.23) in men and 1.42 (95% CI: 1.07 to 1.89) in women. However, the association between BMI and survival differed significantly with stage of COPD. In mild and moderate COPD there was a nonsignificant U-shaped relationship, with the lowest risk occurring in normal-weight to overweight subjects, whereas in severe COPD, mortality continued to decrease with increasing BMI (test for trend: p < 0.001). Similar results were found for COPD-related deaths, with the strongest associations found in severe COPD (RR for low versus high BMI: 7.11 [95% CI: 2.97 to 17.05]). We conclude that low BMI is an independent risk factor for mortality in subjects with COPD, and that the association is strongest in subjects with severe COPD.