A complete survey of the 1426 inhabitants of the Mulia goiter valley in Western New Guinea showed that 80 subjects had presumably congenital defects. Mental deficiency, deaf-mutism and motor defects, in various combinations, were the main abnormalities. In Mulia the defects were only found in subjects born there or in other goiter valleys; the many immigrants from valleys without goiter were unaffected. The geographic relation between defects and endemic goiter was confirmed by data on other valleys both within and outside the goiter area. The defects are therefore regarded as manifestations of endemic cretinism. The incidence of goiter was approximately the same in defectives and nondefectives. So also were the main features of iodine metabolism: low iodine excretion, low serum PBI and high thyroid ¹³¹I uptake. “Classic” cretinous dwarfism was not found among the Mulia defectives, and this seems related to the absence of demonstrable thyroid destruction, and therefore of severe postnatal hypothyroidism. The defects found in Mulia are well known from other descriptions of endemic cretinism. When their pathogenesis is analyzed in relation to prenatal hypothyroidism, it seems inevitable to postulate that deaf-mutism, and perhaps the motor defects as well, results from insufficient supply of the fetus with maternal thyroid hormone. The observed failure of serum PBI to rise in pregnancy is compatible with this supposition.