Because of the limited number of donor organs available, the selection of patients for lung transplantation is crucial. One important issue in setting priorities is the life expectancy without transplantation. The purpose of this study was to estimate survival based on lung function, in alpha 1-antitrypsin deficient patients. Data from the Danish alpha 1-antitrypsin deficiency registry were analysed. The basic element of the analysis was two year intervals, characterized by date of spirometry and known mortality status 2 yrs later. We found a simple exponential relationship between lung function (forced expiratory volume one second (FEV1)) and two year survival on conservative treatment. The formula implies an almost 100% two year survival until FEV1 falls below one third of predicted normal; at this level two year mortality increases and will reach 50% at a FEV1 of 15% predicted. In conclusion, the two year mortality of emphysema patients due to alpha 1-antitrypsin deficiency increases exponentially with decreasing FEV1 and the results imply that only a few patients who underwent lung transplantation would have had a better two year prognosis without this procedure.