PURPOSE: To examine the hidden blood loss (HBL) in treatment of AO type A1-A3 thoracolumbar fractures with three different approaches and to explore the influential factors of HBL among patients after the surgery of internal fixation for thoracolumbar fractures. METHODS: We retrospectively studied 85 patients in treatment of thoracolumbar fractures: 25 patients via percutaneous approach (Group A), 33 patients via paraspinal approach (Group B), and 27 patients via conventional open approach (Group C). The demographic information of the patients was collected. Each patient's preoperative and postoperative hematocrit were recorded and used for calculating the blood loss according to the Gross's formula. The difference of blood loss between the three groups was measured by ANOVA. And influential factors were further analyzed by multivariate linear regression analysis in each group. RESULTS: The average HBL was 240.0 ± 65.1 mL in Group A, 313.7 ± 138.1 mL in Group B, and 382 ± 153.8 mL in Group C. There was statistical difference in the HBL between three groups (P = 0.000). However, multivariate linear regression analysis revealed that HBL of three approaches was not associated with age, gender, body mass index (BMI), percentage of height loss, percentage of height restoration, fracture type, or operation time. CONCLUSION: There was a substantial HBL in the treatment of thoracolumbar fractures, which was neglected by surgeons. Further investigation is necessary to study the risk factors for surgery on HBL in treatment of thoracolumbar fractures.