The early diagnosis of congenital hypothyroidism relies essentially on the biological evidence of the defect. Preliirinary data of a systematic screening of neonates are presented. The values of serum thyroxine (T4) and TSH were determined in the cord blood of 271 infants. In 152 of these children serum TSH was again measured on a finger puncture sample at 5 days of age. Serum T4 was measured by the competitive binding method of Murphy and Patty. Serum TSH was determined by double antibody radioimnunoassay with the TSHk of CEA-IRE-SORIN. Results are as follows : At birth the variability is important for T4 and TSH. On the 5th day the scattering of TSH values is much smaller and the mean is lower than at birth (< 0.001). In agreement with previously reported data we found serum TSH levels of 160 and 500 U/ml in 2 hypothyroid infants younger than 1 month. The striking difference between these high TSH values and the control values observed on the 5th day allow to support the view that this screening procedure is reliable for detecting congenital hypothyr.