Summary 1.A series of forty cases from a group of 500 children submitted for endocrine study formed the basis of this report. 2.The mental or physical retardation, or both, in these children appeared akin to the same, but more severe, findings of sporadic cretinism and marked hypothyroidism. 3.This condition I would term “masked hypothyroidism” since the symptoms are misleading and often go unrecognized unless properly studied and interpreted. Careful questioning frequently discloses a chain of symptoms other than those of which the patient complains. When considered with poor somatic development and retarded osseous unfolding, the findings are significant. 4.Retarded osseous development may be evidenced by the late appearance or poor massing of ossification centers and epiphyseal nuclei. Such retardation was a most important factor in our diagnosis of hypothyroidism—mild as well as marked states. 5.In cases in which the hypothyroidism was of too recent occurrence to be demonstrated roentgenographically, other diagnostic measures were employed: in children over eight years of age, who were cooperative, a basal metabolic test was performed. If it was low, it was considered as confirmatory evidence of hypothyroidism. In children under eight years of age, a basal metabolic rate was usually not informative. 6.Thyroid therapy was efficacious in cases in which, in addition to evidence of physical or mental retardation, there was (1) osseous delay, (2) a low basal metabolic rate, and (3) no response to other forms of treatment for the alleviation of the complaint or combination of complaints. 1.A series of forty cases from a group of 500 children submitted for endocrine study formed the basis of this report. 2.The mental or physical retardation, or both, in these children appeared akin to the same, but more severe, findings of sporadic cretinism and marked hypothyroidism. 3.This condition I would term “masked hypothyroidism” since the symptoms are misleading and often go unrecognized unless properly studied and interpreted. Careful questioning frequently discloses a chain of symptoms other than those of which the patient complains. When considered with poor somatic development and retarded osseous unfolding, the findings are significant. 4.Retarded osseous development may be evidenced by the late appearance or poor massing of ossification centers and epiphyseal nuclei. Such retardation was a most important factor in our diagnosis of hypothyroidism—mild as well as marked states. 5.In cases in which the hypothyroidism was of too recent occurrence to be demonstrated roentgenographically, other diagnostic measures were employed: in children over eight years of age, who were cooperative, a basal metabolic test was performed. If it was low, it was considered as confirmatory evidence of hypothyroidism. In children under eight years of age, a basal metabolic rate was usually not informative. 6.Thyroid therapy was efficacious in cases in which, in addition to evidence of physical or mental retardation, there was (1) osseous delay, (2) a low basal metabolic rate, and (3) no response to other forms of treatment for the alleviation of the complaint or combination of complaints.