A set of identical twins with diabetes mellitus underwent serial endocrine and metabolic studies. Both twins had diabetes mellitus. One of the twins was euthyroid during both study periods while the other twin was hyperthyroid during the first study period and euthyroid during the second study period. The hyperthyroid twin had greater sensitivity to insulin than did the euthyroid twin. Both twins had persistent hypocholesterolemia with concordant pertubations in their polyarcylamide gel lipoprotein electrophoresis patterns. Following the development of hyperthyroidism, one of the twins developed symptomatic sarcoidosis. The sarcoidosis resolved when she later became euthyroid. This study illustrates some of the complex interactions between a disease process and the patient's genetic characteristics.