Serum thyroid hormone levels were measured in women living in an area of severe dietary iodine deficiency in a remote region of Papua New Guinea. Levels of maternal hormone were found to be related to the motor competence of the women's offspring, who were aged between six years and 11 years 10 months at the time of assessment. Motor competence in the children was measured by two tests of manual dexterity which demanded speed and accuracy in performance. A significant correlation was obtained between the children's motor performance and maternal serum thyroxine levels, but not with maternal serum triiodothyronine levels. The implications of the findings for the pathogenesis of some forms of cerebral palsy are considered.