Background: The medial-to-lateral approach is favored by most colorectal surgeons for laparoscopic retroperitoneal dissection and mobilisation of the left colon. The peritoneal access window, i.e. the distance between the inferior mesenteric vein (IMV) and inferior mesenteric artery (IMA) must be large enough to perform the procedure safely and successfully. However, studies investigating the IMA–IMV distance and factors affecting this variable, are scarce. Therefore, we examined the IMA–IMV and D3-IMA distances to determine an anatomical framework on planning and adapting surgical therapy. Basic procedures: The IMA–IMV and D3-IMA distances were retrospectively measured in 230 patients (127 Male/103 Female, Median Age = 54.5) who had undergone pre-operative CT-scanning before laparoscopic left-sided colorectal surgery. Two observers rated the images and interrater reliability was calculated. Subgroup, simple and multiple linear regression analyses were performed in order to detect potential interaction between morphometric variables and IMA–IMV distance. Main findings: We demonstrated a significant correlation between the inferior margin of the duodenum and the origin of IMA. Determination of the IMA–IMV distance was simple and reproducible. Approximately 45% of patients undergoing laparoscopic colorectal procedures had a narrow distance (≤50 mm). There was a sexual dimorphism in IMA–IMV distance, being consistently large in males. There were no other pre-operative factors which predicted whether the peritoneal dissection window for a medial-to-lateral approach was sufficient. Conclusions: Our results provide new data for a better understanding of metric variations in abdominal vascular structures and complement previous observations. In view of our results, we recommend pre-operative measurement of the IMA–IMV before colorectal surgery where the medial-to-lateral approach is planned. Given that a narrow distance may predict a difficult dissection, this factor should be taken into account to determine the optimal surgical approach in each patient.