Previous studies have reported in the literature that fetuin A, released from the liver, may be a useful serum biomarker in the diagnosis of nonalcoholic fatty liver disease (NAFLD). However, results are not consistent in predicting nonalcoholic steatohepatitis (NASH) and fibrosis, which are indicative of progressive NAFLD patients. The aim of this study was to investigate the importance of serum fetuin A in detecting NASH and fibrosis, which are determinants of morbidity, in patients with liver biopsy-proven NAFLD. This observational case-control study included 40 NAFLD patients diagnosed by liver biopsy and 25 healthy volunteer controls. The demographic characteristics of the patients and healthy volunteers were recorded, biochemical tests were done, and the serum fetuin A levels of the participants were determined by the enzyme-linked immunosorbent assay method. Serum fetuin A levels were significantly higher in both the nonalcoholic fatty liver (NAFL) (608.30 μg/mL; p<0.001) and NAFLD (536.58 μg/mL; p=0.008) groups compared to the control group (345.07 μg/mL). However, no significant difference was detected in the NASH group (417.06 μg/mL; p=0.740). When patients with and without fibrosis were compared, the mean serum fetuin A levels of patients with fibrosis were significantly lower (708 μg/mL and 380 μg/mL, respectively; p<0.001). When receiver operating characteristic (ROC) analysis was performed, the best cut-off point for patients with fibrosis was 510.98 μg/mL with 94.7% sensitivity and 66.7% specificity (p<0.001). Serum fetuin A may be a useful biomarker in the diagnosis of NAFLD patients and in predicting liver fibrosis.