No one questions the importance of distinguishing between second- and third-degree burns, and everyone knows the task is difficult and sometimes impossible. Few, however, recognize more than 1 or 2 factors which contribute to mistakes in diagnosis. In the course of experiments performed for other reasons and dealing with histologic studies of several thousand new and healing burns, and in clinical and histologic studies of 26 burned patients, several reasons for our difficulties have become clear. This paper describes certain conditions which cause us to misjudge burn severity and discusses factors other than surface appearance which should influence our judgment. Only thermal burns are considered. Methods The experimental burns were produced on anesthetized young Chester White pigs because their skin resembles human skin more closely than that of any other animal. Radiant energy burns were produced with a modified Army carbon arc searchlight,1 a device which permits the repeated