AIM: To investigate in detail the innervation patterns of the psoas major and iliacus muscles in fetal cadavers.MATERIAL and METHODS: The innervation patterns of 94 psoas major and 94 iliacus muscles belonging to 47 fetuses (28 males, 19 females) aged between the 18th-40th gestational weeks in the laboratory of the Department of Anatomy, Faculty of Medicine were examined. The nerve branches innervating the psoas major and iliacus muscles and the origin levels of these branches were determined. The branches innervating the psoas major and iliacus muscles were classified as L1, L1-L2 level, L2, L2-L3 level, L3, L3-L4 level, L4 and femoral nerve according to the levels at which they originate from the spinal nerves.RESULTS: It was determined that the psoas major muscle was innervated by the ventral branches of the L1-L4 spinal nerves and the femoral nerve, while the iliacus muscle was innervated only by the femoral nerve. The mean number of branches innervating each psoas major and iliacus muscle was 4.69 +/- 2.02 and 3.07 +/- 1.14, respectively. It was observed that the branches innervating the psoas major muscle originated at least at the level between the L1-2 spinal nerves and at most at the level between the L2-3 spinal nerves.CONCLUSION: The spinal nerves innervating the psoas major muscle are at risk of injury during lateral transpsoas surgery, lateral discectomy, and spinal fusion surgery. Therefore, it may be useful for surgeons to know the innervation patterns of the psoas major muscle in surgical interventions in this region. Furthermore, it will be beneficial for surgeons to know the innervation of the iliacus muscle in various surgical interventions on the iliac fossa.