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Background & Aims: Concurrent fatty liver disease represents an emerging challenge in the care of individuals with autoimmune liver diseases (AILD). Therefore, we aimed to validate the ultrasound-based method of controlled-attenuation parameter (CAP) as a non-invasive tool to detect hepatic steatosis in individuals with AILD. Methods: The diagnostic performance of CAP to determine biopsy-proven hepatic steatosis (>5%) was assessed in individuals with AILD (autoimmune hepatitis [AIH], primary biliary cholangitis [PBC], primary biliary cholangitis [PSC], or variant syndromes) who underwent liver biopsy at the University Medical Center Hamburg-Eppendorf between 2015-2020 by calculating the area under the receiver operating characteristic (AUROC) curves. In AIH, the impact of disease activity was evaluated by assessment of CAP upon resolution of hepatic inflammation during follow-up. Results: Overall, 433 individuals with AILD (AIH: 218, PBC: 51, PSC: 85, PBC/AIH: 63, PSC/AIH: 16) were included. Histologically proven steatosis was present in 90 individuals (20.8%). Steatosis was less frequently observed in people with PSC (14%) than in other AILD. CAP values correlated positively with grade of steatosis (ρ = 0.39) and the BMI (ρ = 0.53). In PBC and PSC, the ROC curves defined an AUROC of 0.81 and 0.93 for detecting steatosis at an optimal cut-off of 276 dB/m (sensitivity: 0.71; specificity: 0.82) and 254 dB/m (sensitivity: 0.91, specificity: 0.85), respectively. In AIH, the diagnostic performance of CAP was significantly lower (AUROC = 0.72, p = 0.009). However, resolution of hepatic inflammation under treatment was associated with a significant increase in CAP levels (median [IQR]: +38.0 [6-81] dB/m) and considerably improved diagnostic accuracy (AUROC = 0.85; cut-off: 288 dB/m; sensitivity: 0.67, specificity: 0.90). Conclusions: In PBC and PSC, hepatic steatosis can be reliably detected by applying disease-specific thresholds of CAP. In AIH, the diagnostic accuracy of CAP is moderate at diagnosis, but improves after acute hepatitis has resolved. Impact and implications: Non-invasive estimation of fat content in the liver can be performed with the ultrasound-based method of controlled-attenuation parameter (CAP). Here, we showed that the presence of a concomitant fatty liver is frequent in people with autoimmune liver diseases and we determined disease-specific thresholds of CAP to best predict the presence of a fatty liver. CAP measurement was shown to be a valid tool to detect fatty liver in individuals with PSC and PBC; however, in AIH, CAP had limited accuracy especially when significant inflammatory activity was present in the liver. In the context of substantial liver inflammation, therefore, CAP values should be interpreted with caution, and measurements should be repeated after acute hepatitis has resolved.