Enlargement of the thyroid gland is common in adolescent girls, but there have been no previous reports of the frequency of goiter in pregnant teenagers. This study was undertaken to determine the prevalence of goiter in 309 consecutive pregnant adolescent patients, ages 11 to 19 years, who presented for prenatal care at the University of California (San Diego) Medical Center Teen OB Clinic. Eighteen patients (6%) were found to have a goiter. Subsequent diagnostic tests revealed that 28% of the patients had autoimmune thyroid disorders (chronic thyroiditis or Graves' disease), and the remainder had nontoxic goiters or subacute thyroiditis. Black patients had significantly more thyroid disease than Mexican-American (p less than 0.01) or white patients (p less than 0.005). The incidence of thyroid problems during pregnancy did not differ significantly from that of 600 nonpregnant teenagers who came to the general Adolescent Medicine Clinic for other health problems. Because abnormalities in thyroid function may have potentially adverse implications for mothers and their infants, we recommend careful evaluation of all pregnant teenagers with goiter to assess maternal thyroid function and to provide treatment if necessary.