Mitochondrial dysfunction might play a central role in the pathogenesis of nonalcoholic steatohepatitis (NASH). The aims of this study were to evaluate whether free fatty acid (FFA) transport into the mitochondria or the activity of mitochondria respiratory chain (MRC) complexes are impaired in NASH. In patients with NASH and control subjects, we measured free carnitine, short-chain acylcarnitine (SCAC) and long-chain acylcarnitine (LCAC) esters, carnitine palmitoyltransferase (CPT) activity, and MRC enzyme activity in liver tissue as well as serum concentration of tumor necrosis factor alpha (TNF-alpha), homeostatic metabolic assessment of insulin resistance (HOMA(IR)), and body mass index (BMI). In patients with NASH, the LCAC/free carnitine ratio was significantly increased and the SCAC/free carnitine ratio was decreased. In patients with NASH, the activity of the MRC complexes was decreased to 63% +/- 20% (complex I), 58.5% +/- 16.7% (complex II), 70.6% +/- 10.3% (complex III), 62.5% +/- 13% (complex IV), and 42.4% +/- 9.1% (adenosine triphosphate synthase) of the corresponding control values. Activity of these complexes correlated significantly with serum TNF-alpha and HOMA(IR). Serum TNF-alpha (36.3 +/- 23.1 pg/mL), HOMA(IR) (4.5 +/- 2.38), and BMI (29.9 +/- 3.5 kg/m(2)) values were significantly increased in patients with NASH. In conclusion, activities of MRC complexes were decreased in liver tissue of patients with NASH. This dysfunction correlated with serum TNF-alpha, insulin resistance, and BMI values.