Levels of endothelin (ET)-1 peptide are transiently increased after major physical stress. While studying sequential changes in plasma ET-1 levels during various types of stress, we noticed that the level of plasma ET-1 began to rise 10 days post-operatively in one patient with lung cancer who had undergone a left lower lobectomy. 35 days postoperatively a bronchopleural fistula became clinically manifest. The case is presented and the use of plasma ET-1 as an indicator is discussed.