BACKGROUND: N-acetylcysteine (NAC) is a promising agent for reducing irritability and hyperactivity and enhancing social responsiveness in children with autism spectrum disorders (ASD). This study aims to examine the effects of NAC on cardinal symptoms, eating, and sleeping habits in preschool children with autism. METHODS: The medical records of ASD patients were investigated retrospectively. 37 children with ASD who regularly received oral NAC in two divided doses per day (400-600 mg/day) for 8 weeks were included as the study group. The control group consisted of 21 children with ASD who were recommended NAC but never used it. The initial and second assessment scores after 8 weeks of regular use of the NAC group and control group on the Childhood Autism Rating Scale (CARS), Aberrant Behavior Checklist (ABC), Children Eating Behavior Questionnaire (CEBQ), and the Sleep Habits Questionnaire (CSHQ) were compared. RESULTS: Our findings suggested that oral NAC alleviated the intensity of cardinal autistic symptoms in areas of social withdrawal, interpersonal relationships, body use, listening response, and verbal communication. Corresponding problem behaviors such as irritability, stereotypic behavior, and hyperactivity were reduced. It was determined that there was no difference between the two groups in terms of eating behaviors and sleeping habits. CONCLUSIONS: According to the results, NAC alleviated the severity of cardinal symptoms and reduced problem behaviors in autism. Additional trials with more systematic planning, controlling for confounding effects, and long-term follow-up should be provided in future studies.