A 5-year-old child who weighed 17.5 kg received 50 mg of clonidine. The amount ingested was confirmed by analysis of the suspension administered (clonidine HCl 9.78 mg/mL). To our knowledge, this represents the largest ingestion in a child and the largest ingestion on a milligram per kilogram basis in the medical literature. The child's initial presentation included hyperventilation, an unusual feature of clonidine toxicity. The child was discharged without sequela 42 hours after admission. A serum concentration of clonidine 17 hours postingestion was 64 ng/mL, the highest reported to date in a pediatric patient. The intoxication was traced to a pharmacy compounding error in which milligrams were substituted for micrograms. Increased prescribing of clonidine in young children coupled with the requirement to compound clonidine in a suspension and the narrow therapeutic index suggests that the frequency of severe ingestions in children will increase in the future.