Background. Autism Spectrum Disorders (ASD) may present with a delay in social and communication development, or less frequently, with regression in social and language skills. The reasons for this difference in clinical presentation are unknown, and the regressive symptoms in the second group suggest an acquired process. Methods. We investigated serum autoantibodies in these two types of ASD in a cross-sectional design in a total of 50 children, 24 with autistic regression and 26 with classical ASD according to the DSM-5 criteria. Clinical assessment by the Childhood Autism Rating Scale (CARS) and Ankara Developmental Screening Test (ADST), parental questionnaires consisting of the Aberrant Behavior Checklist (ABC) and Autism Behavior Checklist (AuBC) were completed. Serum samples were tested for anti-neuronal antibodies including anti-N-methyl-Daspartate receptor (NMDAR), anti-contactin-associated protein (CASPR2), anti-leucine rich glioma inactivated 1 (LG1), anti-glutamate type 2-amino-3-propionic Acid (AMPA) 1-2, anti-gamma amino butyric acid (GABA) B, anti-dipeptidyl aminopeptidase-like protein 6 (DPPX) and anti-glutamic acid decarboxylase 65(GAD). Results. Serum anti-GAD antibodies were at detectable levels in five (20.8%) patients with autistic regression, of whom three had 2 to 4-fold increased titers, and in none of the patients with classical ASD. The age of the father at the patient's birth and the duration of autistic regression correlated with anti-GAD IgG levels (P: 0,045, P: 0.855 respectively) in the ASD-regression group. No other antibodies were detected in either group. Conclusions. Our results do not suggest a causative role of anti-neuronal antibodies, but the possibility of an autoimmune process accompanying regressive symptoms in a small subgroup of ASD.