It appears that patients with advanced stages of chronic obstructive pulmonary disease, and particularly emphysema, lose weight and have higher mortality even after controlling for lung function. In the present study, mortality of alpha 1-antitrypsin-deficiency patients PiZ as a function of body mass index (BMI) with control for FEV1, sex and smoking habits was studied. A total of 342 patients participated with a mean follow-up time of 7.6 yr. Ninety patients had BMI under 20 kg m-2, which was the cut-off defining underweight patients. The patients were divided into three groups according to their initial FEV1 % predicted: < 30%, 30-64% and > or = 65%. The underweight patients had significantly higher mortality in the two groups with the lowest FEV1 % predicted. A Cox regression model was applied to control for potential confounders. The risk ratio for the underweight patients was 1.6 (P = 0.03) after controlling for FEV1, age, sex and smoking habits. It is concluded that low body weight is an independent predictor of mortality, but the reason is still unclear.