We report a case of fetal goiter diagnosed by detailed ultrasonography. A 33-year-old woman at twenty weeks of gestation was referred to our hospital for detailed ultrasonography. A fetal goiter was identified. She was receiving propylthiouracil (PTU) 100 mg daily for Graves' disease. Amniocentesis was performed and fetal thyroid function was evaluated as normal. Her recent thyroid function tests were normal, but antithyroid antibodies were positive. The dose of PTU was reduced to 50 mg. However, at twenty six weeks of gestation, maternal thyroid-related autoantibodies became undetectable. A fetal magnetic resonance imaging demonstrated a slight shrinkage of the fetal goiter at 30 weeks. The fetus was delivered vaginally. Thyroid function tests of the neonate were normal, and neonatal goiter was nonpalpable. Fetal goiter is a rare disease. It can be spontaneously resolved by decreasing the maternal dose of PTU.