Neonatal screening has transformed the outlook for children with congenital hypothyroidism. In the Scottish programme which began in 1979, screening is recommended at 4-7 days when the majority of infants are receiving milk. If done earlier, the sample should be repeated for phenylalanine estimation once milk feeding is established. In infants at especial risk of hypothyroidism (e.g. from maternal thyroid disease or anti-thyroid drugs) cord blood should be sent for full thyroid fundion tests. In very low birth weight or sick infants screening at 3 and 10 days, will detect transient neonatal hypothyroidism.If screening is performed within the first week of life it should be possible to start treatment at 11-14 days if there is good communication between the laboratory and clinicians. The starting dose of thyroxine is controversial, with advocates of both medium (7-8 µg/kg/day) and high (10-15 µg/kg/day) dose therapy. A case register with clinical details: age at Guthrie coiledion, notification and start of treatment, diagnostic category, thyroxine dose, growth and intellectual outcome is essential for elevalion, hypothyroxinacmia. audit. If the diagnosis is delayed or doubtful, diagnostic withdrawal of therapy and premalurily re-evaluation is required after the age of 2 years.