Hypothyroidism in patients with diabetes mellitus is usually primary though rarely secondary hypothyroidism has occurred. An 11 6/12 year old white female developed diabetes mellitus at 8 6/12 years of age. She received treatment up to 40 units NPH daily with adequate control and normal growth. Hypothyroidism was diagnosed after a 3 month history of lethargy, constipation, dryness of skin and decreasing insulin requirement to 10 units NPH per day. Physical examination was entirely normal, except for dry skin. Serum levels of free thyroxine, thyroxine, T3 resin uptake, were low as was 131I uptake. Primary hypothyroidism was ruled out by the absence of goitre, absent antithyroid antibodies, low basal TSH levels and increased 131I uptake after TSH administration. Serum TSH levels rose 4-fold in respone to intravenous TRH administration. The patient was treated with 0.15 mg daily of L-thyroxine with very good response. This report describes a patient with juvenile diabetes mellitus and isolated TSH deficiency with hypothyroidism of probably hypothalamic origin, an association not previously described in children.