Maternal thyroid function was evaluated clinically, by reproductive history, and by serial measurements of serum butanol-extractable iodine (thyroxine-like iodine), two before and two after 24 gestational weeks during 1,349 pregnancies. Three percent of the women were hypothyroxinemic. Developmental, intellectual, and motor abilities of progeny born to (Group I) 210 euthyroxinemic, (Group II) 15 hypothyroxinemic given adequate thyroid replacement therapy, and (Group III) 21 inadequately treated hypothyroxinemic women were compared. The groups of mothers exhibited no significant differences in intelligence, years of education, or chronological age. Mean developmental and intellectual scores at eight months, four and seven years of Group II progeny evidenced remarkably consistent similarity to scores of siblings and controls. At each age, mean developmental and intellectual scores were lower for Group III progeny, and motor scores of the latter were lowest. Some progeny of Group II mothers, treated only after 12 or 29 weeks, failed the ball catch and line walk tests; some had strabismus and other ocular disturbances. Could these deficits have originated with maternal hypothyroxinemia during first semester weeks before the thyroid-pituitary axis matures? Now in 1990-1991, early findings fit into the modern concepts of significant maternal gestational transfer of thyroxine to the fetus. The authors encourage prenatal and/or early gestational screening for maternal hypothyroxinemia and urge prescription of adequate thyroid replacement therapy for hypothyroxinemic women.