The clinical course from birth and serial measurements of serum T3, T4 and TSH in an infant with untreated neonatal thyrotoxicosis are reported. The thyroid hormone levels fell exponentially with time at rates very much slower than those previously reported for the maternally-transmitted thyroid stimulating antibody generally thought to cause the disorder. Steady physiological levels of thyroid hormones were achieved after 110 days (serum T3 = 3.4 NMOL/L, T4 = 118 nmol/l). TSH first rose to a measurable level after about 90 days.