Study Design: Retrospective study. Objection: There can be a relationship between degenerative diseases in the spine and hip. Summary of Background Data: Degenerative diseases in the spine and hip may occur concomitantly. This study was done to investigate the cross-sectional area of psoas muscle size and incidence of lumbar disc herniation after unilateral hip arthroplasty. Methods: The data files of patients who were operated on for unilateral hip arthroplasty between January 2014- and 15 May 2021 at the Recep Tayyip Erdogan University Hospital were retrospectively analysed for the relationship between the psoas muscle volume and the incidence of lumbar disc herniation. The patients were divided according to their operated sides. Results: The data files of 48 patients were retrospectively analysed. The patients were divided into two groups according to the operated side of their hip joints. Gender and age differences were not significant, and the mean ages were 68,68 years old in the right hip arthroplasty group, and 69,39 in the left hip arthroplasty group. Conclusions: A complex interaction between the development of lumbar disc herniations and increased contralateral cross-sectional area of the psoas muscle at the L4-5 level was observed in patients operated for unilateral hip arthroplasty. This interaction can be a compensatory mechanism to counteract the spinal imbalance.