The prevalence of autism spectrum disorders (ASD) has increased markedly in recent decades. The exact etiological and pathogenetic factors are not found today. In this study, the authors would like to draw attention to the malfunctioning of the mitochondria as one of the possible pathogenetic factors in the pathogenesis of ASD.Objective. To determine the prevalence of mitochondrial dysfunction among children with ASD.Material and methods. In the study 25 children with ASD and 15 healthy children of the control group aged 1.5–5.5 y.o. (mean age 3.5 y.o.) were examined. There were 18 boys that made up 72% and 7 girls (18%). The serum concentration of lactate and pyruvate was measured in the children.Results. An increase of fasting lactate concentration was found in 18 (72.0%) children with RAS.. In children of this group, moderate lactic acidosis with the exceeded reference values of serum lactate by 25–50% was mainly observed. The serum lactate increasing within 25% was noted in 2 (8%) children, and higher than 50% level was observed in 7 (16.0%) patients. We also found a decrease of the lactate/pyruvate ratio in 19 (76.0%) children with ASD.Conclusions. Children with ASD have severe mitochondrial dysfunction, which can be considered as one of the pathogenetic factors of the regressive forms of ASD. Early detection of mitochondrial dysfunction is critical and requires immediate treatment. We offer a screening determination of the serum lactate, pyruvate and their fasting ratio in all children with ASD.