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Objective. To determine the efficacy of ketogenic diet (KD) therapy on adrenocorticotropic hormone- (ACTH) or corticosteroid-resistant infantile spasm (IS), and identify relevant associated factors.Methods. A prospective controlled study was undertaken at 10 tertiary children's medical centres in mainland China. Participants were non-randomly assigned to KD therapy or control (adjustment of antiepileptic drugs). The primary outcome was the reduction in spasms and remission of hypsarrhythmia at the 16th week, divided into Grade I (spasm-free for at least one week with hypsarrhythmia remission), Grade II (≥50% spasm reduction and/or hypsarrhythmia remission) and Grade III (<50% spasm reduction with hypsarrhythmia).Results. In total,227 patients were recruited and assigned to the KD (135 patients) and control (92 patients) groups. The efficacy in the KD group was superior to that in the control group (Grade I: 13.4% vs. 10.9%; Grade II: 40.7% vs. 20.7%, p=0.025). Patients with a ketogenic ratio <3:1 had a higher rate of Grade I than those with ketogenic ratio ≥3:1 (66.7% vs. 33.3%, p=0.037). No significant correlation was found between the efficacy of KD and level of serum ketosis, aetiology of IS, or age.Significance. The efficacy of KD therapy was superior to adjustment of oral antiepileptic drugs in children with ACTH- or corticosteroid-resistant infantile spasms.