The question of radiological mass screening for congenital dislocated hip is still debated. We have tried to evaluate the cost-benefit ratio of radiological detection at the age of 3-4 months, taking into account the socio-economic cost and radiation risk. Assuming a frequency of this disorder of 1% the average cost of treatment of one case detected by X-ray screening at the age of 3-4 months, including the price of X-ray examinations of 99 normal babies, is 23,374 FF. The average cost of treatment of a case detected when walking (i.e. after 9 months) is 84,230 FF. The cost-benefit ratio is 3.6. In countries where the frequency reaches 2% the cost benefit ratio is 4.57. It also appears from our study that the irradiation of the patient is much smaller when the diagnosis is made earlier. Comparing the slight irradiation delivered to normal infants by this mass screening to the heavy irradiation received by a few individuals whose treatment is started after 9 months, the calculated risk of leukemia or of genetic disorder for the whole population still favours a systematic X-ray film of the pelvis at age 3-4 months. However, if it were decided to make obligatory this mass radiological detection programme during the fourth month of life, this would necessitate a serious effort to train all radiologists to obtain adequate films with the best radiation protection.