OBJECTIVE Sedation is frequently essential for successful MR imaging, and chloral hydrate is the most commonly used drug for this purpose in infants and children. Our experience with these patients suggested that this sedative is less effective in older children, even when administered in high doses. However, no prospective study comparing the efficacy of chloral hydrate sedation for children of different ages undergoing MR imaging has been reported. Accordingly, we performed a study to evaluate the effectiveness and safety of chloral hydrate sedation in children of various ages. METHODS The study included 300 infants and children, 1 month to 11 years old (mean, 3 years), who were given oral chloral hydrate, 100 mg/kg, for sedation before MR imaging. The maximum total dose administered was 2.5 g, which limited the study to children who weighed 25 kg or less. Sedation was considered successful when MR studies were completed and at least 95% of the images had little or no motion artifact. RESULTS Sedation was successful in 273 (91%) of 300 children. It was unsuccessful in nine of the 203 children who were 48 months old or younger (96% success rate) and in 18 of the 97 children who were more than 48 months old (81% success rate). A single-tailed t-test showed that the children in whom sedation was unsuccessful were significantly older than those in whom it was successful to the .0005 level of significance. The failure rate increased steadily for children more than 48 months old. Several failures may also have resulted from lengthy examination times. Adverse reactions to chloral hydrate sedation included hyperactivity (6%), vomiting (4%), and mild respiratory depression (4%). No adverse reaction was severe enough to require hospitalization. CONCLUSIONS The higher failure rate for chloral hydrate sedation in children more than 48 months old suggests that the patient's age is an important limitation to the usefulness of chloral hydrate sedation for children undergoing MR imaging. However, the low rate of adverse reactions makes chloral hydrate a safe drug for sedation of children undergoing MR imaging.