Purpose <p>Full-endoscopic discectomy using the transforaminal approach for herniated nucleus pulposus has seen remarkable advances in recent decades as a minimally invasive spine surgery. Although very rare, this technique has potential risks of dural tears, visceral organ injury as well as exiting nerve root injury. This study aims to define the safe entry range for transforaminal full-endoscopic spine surgery (TF-FESS) at each lumbar level.</p> Materials and methods <p>We retrospectively reviewed multiplanar abdominal CT scans obtained in the prone position from 50 consecutive patients (36 men, 14 women) following discography at our hospital between February 2019 and October 2021. The mean patient age was 44 years (range 15–83). We defined reference lines using reconstructed axial CT images, including the middle space at the intervertebral disc levels from L1–L2 to L4–L5. The following reference lines were established; line B, connecting the center of the rectus abdominis muscle and the center of the paravertebral muscles. We evaluated the trajectory to identify potential interference with the dural canal; line D or visceral organs; line V, the reference line of approach; line R, and the safe entry range for TF-FESS.</p> Results <p>The safe entry range at the upper lumbar levels was characterized by a shorter distance and a steeper angle, increasing the risk of interference with the dural canal. A similar trend was observed regarding interference with the visceral organs, particularly the kidneys, which are of concern at the upper lumbar levels. Considerable caution is needed when performing TF-FESS at upper lumbar levels.</p> Conclusions <p>Endoscopic spine surgeons should be cognizant of these findings and keep them in mind during puncture to avoid interference with the dural canal and visceral organs, particularly when performing TF-FESS at upper lumbar levels.</p>