Anteroposterior X-ray of lumbar spine of 52 newborn infants(29 male, 23 female) were taken to estimate lengthof lumbar vertebrae comparing with several clinical criteria of fetalmaturity such as body length, gestationperiod and body weight. Those were evaluated and their correlation were studied. 1. In 46 out of 48 cases whosebody length were 47 cm or more, estimated value of lumbar vertebral length were 46mm or more(95.8%). 2. In 46cases of 49 newborn infants whose gestation period were 37 weeks or more, the length of lumbar vertebrae were 46mmor more (93.9%). 3. In 46 cases of 47 newborn infants whose body weight were 2,500 gm or more, the length oflumbar vertebrae were estimated 6mm or more (97.9%). 4. Among 49 cases of newborn infants who were estimated oflumbar vertebral length of 46mm or more, clinically matured newborn infants were 46 to 47 cases (93.9-95.9%). As amean of radiological detection of fetal maturity, the length of lumbar vertebrae has close correlation withclinical criteria, but further studies for prenatal estimation should be expected for practical use.