Introduction This study was undertaken in an attempt to clarify some of the problems which have arisen from the marked variability in the ocular changes associated with thyroid dysfunction. The ocular signs may be abrupt or insidious in onset; mild or severe; unilateral or bilateral; more advanced about one eye than the other. They may appear long before any evidence of thyrotoxicosis, with the onset of thyroid dysfunction or following its treatment. In addition, the pathogenesis of these ocular signs is poorly understood, and there is no agreement as to whether they represent one or two disease processes. A clearer understanding of their nature is not only of academic interest, but also of practical importance for the clinician often has difficulty in interpreting the significance of these ocular findings. This report is divided into two portions: the first from the laboratory and the second from the clinic. Both parts, however,