Most of the hyperthyroidism seen in association with pregnancy is Graves' disease. The best treatment is prevention. For most patients there is an opportunity to treat the hyperthyroidism decisively with radioiodine or surgery before the patient becomes pregnant. Pregnancy complicated by hyperthyroidism is often a consequence of the conscious decision to treat hyperthyroidism in women in the childbearing years with antithyroid drugs. Propylthiouracil (PTU) is the preferred treatment for hyperthyroidism in pregnancy, but it does cross the placenta and can induce fetal goiter, with mental and physical retardation. Hence, the lowest possible PTU dose should be used. One should aim for high normal or slightly elevated thyroid function in the mother. Patients should be followed at 3-week intervals if progress is satisfactory, more often otherwise. Thyroid function should be monitored by the free T4 assay. PTU dosage should be reduced progressively in anticipation of the customary steady amelioration in the hyperthyroidism that occurs in later stages of pregnancy. Since pregnant hyperthyroid patients are sometimes irresponsible and continue PTU without supervision, PTU prescriptions should be limited to the amount required for the time until the next scheduled visit. For about one third of patients, PTU can be discontinued in the second half of the pregnancy. After the pregnancy is terminated, persistent or recurrent hyperthyroidism should be treated definitively to prevent another episode of pregnancy complicated by hyperthyroidism.