Background <p>Simple, accurate methods are required for diagnosing metabolic dysfunction-associated steatotic liver disease (MASLD). Although the fatty liver index (FLI) is a simple and useful biomarker for steatotic liver disease (SLD), its optimal cutoff values for diagnosing MASLD and MASLD with increased alcohol intake (MetALD) remain unclear.</p> Methods <p>This cross-sectional study included 2512 adults undergoing health checkups with abdominal ultrasonography (AUS) and vibration-controlled transient elastography (including control attenuation parameter [CAP]). We used CAP 268&#xa0;dB/m as the cutoff for SLD diagnosis. We analyzed the diagnostic performance of FLI for MASLD and MetALD. Optimal cutoff values were determined using area under receiver operating characteristics curve (AUROC) and Youden index.</p> Results <p>Among 2512 individuals studied, 956 had SLD, including 648 with MASLD, 231 with MetALD, and 67 with alcohol-associated liver disease. The distribution of FLI values (&lt; 30, 30–60, &gt; 60) was 46%, 31%, and 23% in males and 83%, 12%, and 5%, in females. For MASLD, the AUROC and optimal FLI cutoff values were 0.786 and 26.7. When analyzing by sex, these values were 0.729 and 26.9 for males and 0.886 and 19.2 for females. For MetALD, the corresponding values were 0.835 and 34.5. When analyzing by sex, these values were 0.764 and 44.4 for males and 0.95and 30.8 for females. Diagnostic agreement rate between AUS and CAP was 78.3% in all, and 74.9% in males and 84.1% in females.</p> Conclusion <p>The optimal FLI cutoff for MetALD was higher than for MASLD, with noticeable sex differences observed.</p>