ABSTRACT Objective Hip and knee OA often occur successively. However, little is known about the relationship between hip and knee OA in unilateral knee OA patients. The aim of the study was to explore the correlation between unilateral knee OA and bilateral hip degeneration to determine the impact unilateral knee OA has on the severity of hip degeneration. Methods This retrospective case–control study enrolled 162 patients who were presented with unilateral knee osteoarthritis and hospitalized in Zhongshan hospital, Fudan University, from January 2016 to December 2018. The severity of affected knee joint and bilateral hip joint was evaluated by using the Kellgren–Lawrence (KL) score. Variates, such as gender, age, body mass index (BMI) and Knee Society Score (KSS) at admission, were also recorded. Differences in the severity of OA between ipsilateral, contralateral hip joints and affected knee joints were accessed with chi‐square tests. Single‐factor and multi‐factor logistic regression were applied to determine the risk factors for hip OA in patients with unilateral knee OA. Results Gender have no impact on KSS, the length of disease, and BMI of patients. In contralateral hip joints, there was significant difference in the proportion of patients with severe hip degeneration (KL score ≥ 2) compared to those with mild hip degeneration (KL score 5 years) (OR 3.030 [95% CI 1.476 to 6.220]; p