The authors report three children who exhibited developmental learning disabilities (DLDs) associated with behavioral disturbances, such as attention deficit, hyperactivity, and autistic features. The thyroid function tests performed as a part of routine endocrinologic evaluation of children with DLDs revealed a hormonal profile consistent with hyperthyroidism. These children had no systemic signs of hyperthyroidism. Treatment with neomercazole resulted in good control of their hyperkinetic behavior and subsequent improvement in language function attributable to an increased attention span, thereby facilitating speech therapy. Although routine screening of all children with DLDs for thyroid dysfunction may not be cost-effective, selective screening of children with familial attention-deficit hyperactivity disorder and those with attention-deficit and hyperactivity in association with DLDs and pervasive developmental disorders appears to be justified.