Weight changes in 100 patients suffering from advanced chronic obstructive pulmonary disease who presented obvious CO2 retention at least once, were studied. Each emaciation of more than 10% of the initial weight was regarded as weight loss. According to the initial weight the patients were divided into 3 groups normal, underweight, or overweight, as compared with the ideal weight. Weight loss was comparable in the 3 groups and amounted on an average to 20% of the initial weight. It was particularly pro- nounced during the 1st year after the start of weight loss. The patients who subsequently died presented a further loss of weight during the following years. Cumulative death rates showed a mortality of 30% 3 years after the onset of weight loss and 49% 5 years after; these rates are significantly higher than those in patients without weight loss. Only 1/5 of the patients died without presenting weight loss. A definite relation was observed between weight loss and the 1st attack of heart failure, which occurred, on the average, 21 months after the onset of weight loss. One third of the patients presented heart failure within 6 months after the onset of weight loss. In the over-all group of patients, no obvious correlation between weight loss and lung infection tests could be established, because of the spreading of the values. In a restricted group composed of the same patients, a significant deterioration of some functional data was found. This was most pronounced for the 1-sec. forced expiratory volume in patients who had lost weight as well as inthe others. A decreased caloric intake is an important cause of weight loss, as demonstrated by dietary studies.