Background: Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder, the etiology of which has not been clearly determined yet. There is increasing evidence that synaptic and dendritic changes are involved in the etiology of ASD. The aim of this study is to determine whether serum Thrombospondin-1 and Thrombospondin-2 differ between ASD patients and healthy controls. This study also investigates possible correlations between clinical symptomatology of ASD and serum Thrombospondin-1 and Thrombospondin-2 levels. Method: A total of 44 children with ASD and 21 healthy controls under 6 years of age were included in the study. Symptom severity and behavioral problems among children with ASD were evaluated by using Childhood Autism Rating Scale and Abnormal Behavior Checklist. Serum levels of Thrombospondin-1 and Thrombospondin-2 were measured by using commercial enzyme-linked immunosorbent assay kits. Result: No statistically significant differences were found between the two groups in terms of serum Thrombospondin-1 and Thrombospondin-2 levels. In addition, no correlation was determined between Thrombospondin-2 levels and clinical symptomatology and severity of ASD. However, the Thrombospondin-1 level was found to negatively correlated with the total score of Childhood Autism Rating Scale, inappropriate speech and stereotype subscale scores of Aberrant Behavior Checklist scale. Conclusion: Thrombospondin-1 might have a potential role in the etiopathogenesis of ASD. Further studies are required to clearly elucidate the association between Trombospondin-1 and ASD.