Two cases are described of surgery for spondylolisthesis requiring prolonged knee-chest position (5 h 20 min and 4 h 30 min); acute renal failure with anuria occurred early in one case (within the first 24 h postoperatively), and later in the other case (on the 8th day). The diagnosis of rhabdomyolysis was made on the increase of CPK and myoglobin blood levels. Post-haemodialysis evolution was satisfactory. The possible mechanism was muscle compression against the rests. Diagnosis must be made quickly; the only treatment is early fasciotomy, with the supplying of alkali to prevent acute renal failure. It would appear that the knee-chest position can be kept a maximum of 3 h without any problem.