BackgroundVery little has been published about the diagnostic performance of MRI in total hip arthroplasty (THA) infection.PurposeTo determine the diagnostic performance of conventional MRI features and of new lymph nodal indices to identify infection in patients with failed THA.Study TypeRetrospective.PopulationIn all, 119 patients (66 females; age 66.9 ± 12.4 years) with failed THA.Field Strength/SequencesMetal artifact reduction sequence (MARS) protocol including short tau inversion recovery and turbo spin‐echo T1‐ and T2‐weighted sequences at 1.5T.AssessmentPatients underwent pelvis MRI prior to failed THA revision. MRIs were reviewed to identify periprosthetic bone destruction, soft‐tissue mass, effusion, synovitis, lamellated synovitis, extracapsular edema, fibrous periprosthetic membrane, bone edema, and extracapsular collection/sinus tract. The number and maximum diameter of inguinal, obturator and iliac lymph nodes of the affected hip were assessed and normalized to those of the unaffected hip to calculate the ratio of nodal size (RNS), ratio of node number (RNN), difference of nodal size (DNS), and difference of node number (DNN).Statistical TestsThe Mann–WhitneyU‐and chi‐square test were used. Diagnostic performance of indices and odds ratios (OR) were calculated.ResultsRNS, RNN, DNS, and DNN indices were significantly different (P= 0.000) between infected and noninfected THA, with accuracies ranging from 84.8% (RNS) and 93.1% (RNN). All other MRI features were significantly more prevalent in infected THA (P ≤ 0.002), except bone destruction, periarticular soft‐tissue mass, and fibrous membrane (P ≥ 0.031). Sensitivities ranged from 7.9% (soft‐tissue mass) to 76.3% (effusion/bone edema), specificity from 45.7% (bone destruction) to 97.5% (synovitis/lamellated synovitis), accuracy from 49.6% (bone destruction) to 81.5% (synovitis), OR from 0.261 (soft‐tissue mass) to 35.550 (synovitis).Data ConclusionConventional MRI features have limited accuracy to differentiate septic and aseptic THA failure. Lymph nodal indices, particularly those related to nodal number, may represent biomarkers of THA infection.Evidence Level3Technical Efficacy Stage2