Transient congenital hypothyroidism due to maternal thyrotrophin binding inhibitor immunoglobulin (TBII), a thyroid-stimulating hormone (TSH)-receptor blocking antibody, is described in three male siblings born to a mother with autoimmune thyroiditis. These cases are believed to be the first described in Australia. The first child was found to have a serum TSH of 565 mU/L and had a negative thyroid scan when presented for neonatal screening. He was treated with thyroxine but became thyrotoxic at 3 months of age when he was on a dosage of 85 micrograms/m2 of body surface area. He was euthyroid 6 months after discontinuation of therapy. Nine years later a second hypothyroid sibling was born, with a serum TSH of 709 mU/L on day 4. Both mother and child were demonstrated to be strongly positive for TBII. Again this child was able to cease therapy by the age of 9 months. A third sibling, also TBII positive, was born 12 months after the second. His TSH was 90 mU/L and his serum thyroxine (T4) was 169 nmol/L. On this occasion, thyroid stimulation-blocking antibody was found to be present in the serum of both mother and child. Thyroxine therapy was ceased at 1 month. The family present a picture of varying degrees of transient neonatal hypothyroidism due to the transplacental passage of a maternal receptor blocking antibody. The condition is self-limiting, resolving when the immunoglobulin is cleared from the infant's circulation.