Problem statement: Erythrocyte deformability represents intrinsic property of erythrocytes to pass through the narrow capillaries in the microcirculation. It is crucial for the oxygen delivery to all tissues in the human body. The data available in medical databases indicate that erythrocytes in autism spectrum disorder (ASD) may be altered. The aim of the present study was to determine erythrocyte deformability in Slovak children with ASD and correlate it with clinical features. Methods: Our cross-sectional study involved 83 individuals with diagnosed ASD that was determined using Autism Diagnostic Observation Schedule – second edition (ADOS-2), and the Autism Diagnostic Interview-Revised (ADI-R). Erythrocyte deformability was determined by filtration method. Software GraphPad Prism 7 was used for the data analysis. The study was approved by the Ethics Committee of Faculty of Medicine, Comenius University and University Hospital in Bratislava, Slovak Republic. Written informed consent was obtained from all the subjects or their caregivers. The study conformed to the code of ethics stated in the Declaration of Helsinki. Results: We have found significant negative correlation of erythrocyte deformability with one subdomain of the ADI-R diagnostic tool, specifically: C2 – apparently compulsive adherence to nonfunctional routines or rituals (P=0.01). The correlation between erythrocyte deformability and domains A and B, i.e. qualitative abnormalities in reciprocal social interaction and qualitative abnormalities in communication, was not observed. Conclusion: Abnormalities in erythrocyte deformability may at least partially be involved in pathomechanisms of ASD and contribute to its clinical manifestation. Further research is necessary in order to bring more data and to establish if erythrocyte deformability can serve as prognostic biomarker in ASD.