Liver biopsy is still the gold standard in the staging of nonalcoholic fatty liver disease (NAFLD), which is the most common chronic liver disease worldwide. However, being an invasive method, liver biopsy has limited use in clinical practice. The aim of this study was to determine the relationship between serum levels of cytokeratin 18 (CK-M30) and N-terminal procollagen III propeptide (PIIINP) in patients with biopsy-proven NAFLD. The study was carried out on volunteers, including both healthy individuals and patients pre-diagnosed with NAFLD. The liver biopsies were re-assessed by applying the Steatosis, Activity, Fibrosis/Fatty Liver Inhibition of Progression (SAF/FLIP) algorithm. At the end of the study, frozen serum samples (−80 °C) were analyzed using commercial kits. CK18-M30 and PIIINP levels significantly differed in all study groups. There was no significant correlation between serum levels of CK18-M30 and PIIINP in healthy individuals but there was a significant positive correlation between CK18-M30 and PIIINP levels in NAFLD (NAFL-nonalcoholic steatohepatitis (NASH)) groups. CK18-M30 was better than PIIINP at distinguishing between NAFL and NASH. The results obtained for biopsy-proven NAFLD demonstrated that both PIIINP and CK18-M30 were partly associated with histological parameters and could aid in distinguishing between NASH and NAFL.