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