The intervertebral distance in 31 prolapsed discs under traction increased on the average by 1.34 mm, but in normal discs 0.87 mm. Of 47 prolapsed discs without traction, 29 had negative intradisc pressure, 10 zero, and 8 positive. The total average value of the pressure before traction was -1.3 kPa. Under lumber traction, the pressure decreased in 30 discs, unchanged in 8, and increased in 9, with the average value being -3.2 kPa. The intradisc pressure in 7 normal discs was contrary to the pressure in the prolapsed disc. The difference between the two groups was highly significant (P < 0.01). The pressure unchanged when the intervertebral distance of prolapsed disc unchanged under traction, but the pressure decreased as the distance increased in most cases under traction.