The study aims to examine functional and structural network organisation and changes in the brains of young children with autism spectrum disorder (ASD), from the age of the first symptom onset (at 18 to 24 months) through the full symptom manifestation at age 4 to 5 years, using a multimodal magnetic resonance imaging (MRI) approach (with MRI data acquired during natural sleep). Participants include over 80 toddlers with a suspected diagnosis of ASD and 47 typically developing toddlers. Participants were recruited through referral from health providers, community programs and advertisements in Southern California, United States of America. Baseline assessment, including autism assessment and MRI was conducted from 2016 to 2020 and participants are followed up after around 1.5 and 3 years.