With the current limited drug therapy for the core symptoms of autism spectrum disorder(ASD),we herein report a randomized,double-blind,placebo-controlled trial to investigate the efficacy,safety,and potential neural mechanism of bumetanide in children with ASD aged 3-6 years old.A total of 120 children were enrolled into the study and randomly assigned to either 0.5 mg bumetanide or placebo.In the final sample,119 children received at least one dose of bumetanide(59 children)or placebo(60 children)were included in the final analysis.The primary outcome was a reduction in the Childhood Autism Rating Scale(CARS)score,and the secondary outcomes were the Clinical Global Impressions Scale(CGI)-Global Improvement(CGI-Ⅰ)score at 3 months and the change from baseline to 3-month in the Autism Diagnostic Observation Schedule(ADOS).Magnetic resonance spectroscopy(MRS)was used to measure γ-aminobutyric acid(GABA)and glutamate neurotransmitter concentrations in the insu-lar cortex(IC)before and after the treatment.As compared with the placebo,bumetanide treatment was significantly better in reducing the severity.No patient withdrew from the trial due to adverse events.The superiority of bumetanide to placebo in reducing insular GABA,measured using MRS,was demon-strated.The clinical improvement was associated with a decrease in insular GABA in the bumetanide group.In conclusion,this trial in a large group of young children with predominantly moderate and sev-ere ASD demonstrated that bumetanide is safe and effective in improving the core symptoms of ASD.However,the clinical significance remains uncertain,and future multi-center clinical trials are required to replicate these findings and confirm the clinical significance using a variety of outcome measures.