Comprehensive screening programs to control congenital hypothyroidism (CH), a preventable form of mental retardation, are being considered by some public health agencies. The proposed programs would test neonates' blood for thyroxine and, if warranted, provide follow-up testing and therapy. The estimated cost of detecting a single case of CH is $9,300, which includes specimen collection, laboratory analysis, and retesting of border-line cases. The present value of the treatment costs of CH adds $2,500 per case, a total cost of $11,800 per case detected and child treated. The economic benefits (averted costs of institutionalization and special education and increased productivity of the affected person) are estimated to have a present value of $105,000 per case, yielding a cost-benefit ratio of 1:8.9.