Background and Aims:  Non‐alcoholic fatty liver disease has long been accepted as benign; however, recent evidence suggests that the disease may progress to cirrhosis and hepatocellular carcinoma, although the natural course of the disease is still unclear. This study was designed to comparatively evaluate electron microscopic features of non‐alcoholic fatty liver (NAFL) and non‐alcoholic steatohepatitis (NASH).

Methods:  Quantitative and semi‐quantitative ultrastructural evaluations were performed on liver biopsies from 23 patients, 10 with NAFL and 13 with NASH.

Results:  No statistically significant difference was noted between NAFL and NASH patients in ultrastructural features of hepatocytes including megamitochondria, intramitochondrial crystalline inclusions, mitochondrial matrix granules, foamy cytoplasmic appearance, electron‐lucent and glycogen‐containing nuclear regions, lipofuscin granules, or an increased frequency of vesicles containing electron‐dense material in peribiliary Golgi zone; however, the mitochondrial diameter was significantly higher in the NASH patients. Intercellular distance and microvilli between hepatocytes, collagen and electron‐dense material accumulation in the space of Disse, electron‐dense material accumulation and microvillus density in bile canaliculi did not differ significantly between the groups.

Conclusions:  Our data show that, although NAFL and NASH can be distinguished by their distinct light microscopic features, ultrastructural characteristics are similar, which suggests that NAFL may also have the potential to progress to fibrosis and cirrhosis like NASH.