Forty one patients with 51 pregnancies form the basis of this study of etiological factors and reciprocal effects of pregnancy and thyrotoxicosis. Pregnancy influences the course of thyrotoxicosis and may modify its diagnostic aspects, clinical course, variations in laboratory parameters and treatment. The effect of thyrotoxicosis on pregnancy may lead to abortions, premature labour and/or toxemia. It may sometimes influence the products of conception. The essential point appears to be to seek the minimal dose of antithyroid drug capable of controlling thyroid disease without impairing fetal thyroid function.