The clinical manifestations of various degrees of mental retardation, spastic diplegia, and deaf mutism are known as the neurologic type of endemic cretinism (EC), occurring in countries with high goiter endemicity. Maternal iodine deficiency has been established as the major cause in EC, whereas a genetic predisposition has not been well-documented. Genetic data on 70 families with EC from Highland Ecuador are reported. A segregation analysis of 49 fully classified families yielded an estimate of P = 0.245 (var [P] = 0.00167). Half-sibs were all unaffected and no significant birth order effect was observed among 101 probands. The data indicate an autosomal recessive predisposition as a major etiological factor. Because the neurologic type of EC represents a defined section of the spectrum of iodine deficiency disorders (IDD), the term fetal iodine deficiency disorder (FIDD) rather than cretinism is suggested. The clinical findings in 70 patients were used to delineate the minimal diagnostic criteria of FIDD.