Chloral hydrate is commonly used to sedate infants for pulmonary function tests and other investigations. However, sedation is generally not recommended for infants with acute wheezing illnesses. The commonly used dose range exceeds the maximum recommended dose; however, the effects of this dosage regimen during pulmonary function testing have not been studied. The present study shows that 70-100 mg/kg of chloral hydrate, a dose commonly used to sedate infants for pulmonary function testing, causes a fall in arterial oxygen saturation and a decrease in clinical score of infants recovering from acute viral bronchiolitis, but not in infants with clinically stable cystic fibrosis. These findings suggest that wheezy infants with baseline SaO2 less than or equal to 94% are more susceptible to central respiratory depression following sedation with chloral hydrate. The results of the present study raise serious doubts about the advisability of sedation with chloral hydrate, in the currently used doses, in wheezy infants.