The following lines are intended to give an explanation of Kierkegaard's Thorn in the Flesh. They may be of interest not only to the biographer, but also to the student of Kierkegaard's terminology. We shall consider contemporary interpretation and Kierkegaard's own interpretation of the term “the thorn in the flesh,” review later scholarly theories, and finally present our own solution based upon a comparative study of passages from the Diaries, reminiscences of Kierkegaard preserved by his acquaintances, and extracts illustrating contemporary medical opinion.