BACKGROUND: Nutritional status, weight loss and cachexia have important prognostic implications in patients with chronic obstructive pulmonary disease (COPD). Body mass index (BMI) has been implicated in COPD risk assessment, but information is mostly limited to composite scores or to patients with stable disease. We aimed to analyse the association between BMI and mortality in acute exacerbation of COPD. METHODS: This retrospective survey included 968 patients hospitalized due to acute exacerbation of COPD at the University Clinic Golnik from February 2002 to June 2007. Vital status was ascertained with Central Population Registry, and database was censored on November 1, 2008. RESULTS: Median BMI was 25.08 kg/m(2) (interquartile range, 21.55-29.05 kg/m(2)) and 210 patients (22%) had BMI < 21 kg/m(2). During median follow-up of 3.26 years (1.79-4.76 years), 430 patients (44%) died. Lowest mortality was found for BMI 25.09-29.05 kg/m(2). When divided per BMI decile, mortality was lowest for BMI 25.09-26.56 kg/m(2) (33%). In univariate analysis, BMI per quartile and BMI per unit increase were predictive for all-cause mortality. In an adjusted model, BMI per 1 kg/m(2) unit increase was associated with 5% less chance of death (hazard ratio 0.95, 95% confidence interval 0.93-0.97). CONCLUSIONS: Low BMI < 21 kg/m(2) is frequent in patients hospitalized due to acute exacerbation of COPD. Higher BMI was independently predictive of better long-term survival. A better outcome in obese patients compared to normal weight is in contrast to primary prevention data but concurs with observations of an obesity paradox in other cardiovascular diseases.