Aim: There are increasing reports about the potential role of kynurenine pathway metabolites in autism spectrum disorder (ASD). Early childhood is a very crucial period of time for the etiopathogenesis of ASD and previous studies reported an age-dependent alteration in kynurenine metabolism. However, no study specifically examined kynurenine metabolites in very young children with ASD. This study aimed to investigate kynurenine pathway metabolite levels, kynurenine pathway enzyme activities and neuroprotective index (kynurenic acid/3-hydroxykynurenine ratio) in toddlers and preschool children with ASD. Materials and Methods: A total of 68 children with ASD and 44 healthy controls aged between 18 and 60 months were included in this study. Serum levels of kynurenine pathway metabolites were determined by liquid chromatography-mass spectrometry/mass spectrometry system. Results: Serum 3-hydroxykynurenine and kynurenic acid concentrations were significantly higher in the ASD group than in the control group, whereas serum 3-hydroxyanthranilic acid concentrations were significantly lower. Conclusions: These findings showed that the kynurenine pathway may play a role in the etiopathogenesis of ASD in early childhood.