Because of the vulnerability of preterm infants and the important functions of sleep to health and recovery, researchers have used the assessment of sleep-wake states extensively in research with preterm infants. Sleep-wake states have been used to evaluate the safety and effectiveness of interventions, as an outcome measure of interventions to promote sleep-wake state development and as a predictor of developmental outcome. The usual observations of infant behavior and polysomnography methods of sleep-wake state assessment are very resource-intensive, therefore limiting their widespread use. The purpose of this study was to examine the validity of a new, less resource-intensive, instrumented measure of infant sleep-wake states using infant respiration, body movements, and rapid eye movements against an accepted behavioral observation measure. The instrumented measure demonstrated significant reliability for assessing sleep states but was less reliable for waking states. © 2005 Elsevier Inc. All rights reserved.