Abstract Sedatives with a long duration are required for pediatric magnetic resonance imaging (MRI) in the emergency department. The success rate of chloral hydrate for pediatric sedation is 80% to 100% according to some studies. However, the success rate varies by age, weight, and underlying disease. To identify factors affecting the success rate, we compared the chloral hydrate sedation success rate and adverse event rate by age, weight, and underlying disease. Compared with patients in the failure group, patients in the successful group were younger (23.8 vs 36.9 months, P < .01) and weighed less (11.4 vs 14.4 kg, P < .01). No differences in neurological problems, reasons for MRI, or adverse events were observed between the 2 groups. Patients 18 months old had a success rate greater than 95%, but the success rate decreased in children older than 18 months. The adverse event rate was about 10% in patients 18 months old and increased to 20% in patients older than 36 months. Patients 24 months of age who had a neurological problem (seizure disorder or developmental delay) had a success rate greater than 95%, but the adverse event rate increased after 24 months of age. Chloral hydrate sedation was appropriate for pediatric MRI in patients younger than 18 months. Although we observed no fatal adverse events, it is necessary to monitor patients until full recovery from sedation.