This report describes the occurrence of cephalothoracic lipodystrophy in one of 7-yr-old identical twin sisters. The affected twin had classical loss of sc fat from her face, upper arms, and trunk as well as associated hypocomplementemia, microscopic hematuria, and a borderline oral glucose tolerance test without hyperinsulinism. The unaffected twin had a normal urinalysis, serum complement, and oral glucose tolerance. Both twins, when challenged iv with LRH or TRH, showed appropriate FSH and LH or TSH and PRL responses, respectively. This report, in conjunction with another similar twin pair recently described in the German literature, makes a simple, single gene genetic etiology untenable and supports the view that cephalothoracic lipodystrophy in an acquired disease.