CONTEXT: Nonimmune fetal goitrous hypothyroidism is a rare condition that can induce obstetrical and/or neonatal complications and neurodevelopmental impairments such as those still seen in some patients with congenital hypothyroidism. Prenatal treatment to prevent these adverse outcomes is appealing, but experience is limited and the risk to benefit ratio controversial. OBJECTIVE: The objective of the study was to evaluate the feasibility, safety, and effectiveness of intrauterine l-thyroxine treatment in a large cohort with nonimmune fetal goitrous hypothyroidism. DESIGN: This was a retrospective study of 12 prenatally treated fetuses diagnosed between 1991 and 2005 in France. METHODS: During pregnancy, goiter size and thyroid hormone levels were compared before and after prenatal treatment. At birth, clinical, laboratory, and ultrasound data were evaluated. RESULTS: Prenatal treatment varied widely in terms of l-thyroxine dosage (200-800 microg/injection), number of injections (one to six), and frequency (every 1-4 wk). No adverse events were recorded. During pregnancy, thyroid size decreased in eight of nine cases and amniotic-fluid TSH levels decreased in the six investigated cases, returning to normal in four. However, at birth, all babies had hypothyroidism, indicating that intraamniotic TSH levels did not reliably reflect fetal thyroid function. CONCLUSION: Our data confirm the feasibility and safety of intraamniotic l-thyroxine treatment for nonimmune fetal goitrous hypothyroidism. Although goiter size reduction is usually obtained, thyroid hormone status remains deficient at birth. Amniocentesis seems inadequate for monitoring fetal thyroid function. Further studies are needed to determine the optimal management of this disorder.