Objective: Although there are many reports on the anatomy of the lumbar plexus and knowledge of the morphological aspects of this plexus is of practical importance during the surgical and anaesthetical interventions, the thickness and length of the lumbar nerves and roots have not been studied in detail. Methods: The anatomy of the lumbar plexus and lumbar roots was investigated through dissection of 10 human anatomic specimens. The lumbar plexus was dissected bilaterally in 10 cadavers with the posterior approach and the lumbar roots and plexus were exposed entirely in all specimens. The course and sizes of the lumbar nerves and roots in the posterior wall of the abdomen relevant to soft tissue and bony landmarks are described. Six lumbar nerves and four lumbar roots were included in this work. Results: According to the morphometric measurements, L1 is the thinnest (4.1 mm) and L4 is the largest (5.5 mm) root of the lumbar plexus. The iliohypogastric nerve is the longest (210 mm) and the ilioinguinal nerve is the thinnest (1.2 mm) nerve of the lumbar plexus. Conclusions: The lumbar plexus is located in the substance of the psoas major muscle in the posterior abdominal wall. Meticulous dissection and thorough knowledge of the anatomy are required for the appropriate exposure of the lumbar plexus.