Objective To determine the total blood loss and hidden blood loss associated with surgery for lumbar spinal stenosis and to identify risk factors for blood loss. Methods From September 2002 to July 2006, the cl inical data from 138 patients with lumbar spinal stenosis undergoing initial operation were analysed prospectively. There were 44 males and 94 females, aging 56-78 years (mean 66.7 years). A simple posterior lumbar spinal decompression was used in 26 cases;posterior spinal canal decompression, interbody distraction Cage, and bone graft between transverse process was used in 54 cases; pedicle screw fixation, posterior decompression and bone graft between transverse process was used in 32 cases; posterior decompression, pedicle screw fixation, interbody Cage, and graft between transverse process was used in 26 cases . Before operation, 23 patients took aspirin, and after operation 15 patients had gastrointestinal bleeding. Intraoperative blood loss was calculated by the aspirator and observed blood loss intraoperation. The whole estimated blood loss was calculated according to the level of hemoglobin, blood volume and blood transfusion at the time of admission and after 3 and 4 days of operation. Results The blood loss intraoperation was (485.51 ± 143.75) mL. The estimated blood loss was (1 218.60 ± 306.86) mL, which was significantly higher than the intraoperational blood loss (P lt; 0.001). There was significant difference between the estimated blood loss and observed blood loss during surgeries (P lt; 0.001). There were significant differences in the estimated blood loss and observed blood loss during surgery between patients treated with aspirin and without aspirin (P lt; 0.001), between patients with gastrointestinal bleeding and whiout gastrointestinal bleeding (P lt; 0.001). Conclusion The total blood loss after surgery for lumbar spinal stenosis is much greater than that of observed intra-operation. The type of surgery, treatment with aspirin and gastrointestinal bleeding or ulceration can all independently increase blood loss.