A mixed general linear model analysis of the development of sleep-wake stales was conducted on 37 high-risk preterm infants and replicated with a second cohort of 34 infants. Most dependent variables showed significant development over the preterm period: active sleep decreased, and active waking, quiet waking, and the organization of active sleep and quiet sleep increased over the preterm period in both cohorts. The amount of quiet sleep also increased over age, but this change was significant only for Cohort 1. Seven infant characteristics used as covariates had only minor effects. There were no significant differences in the developmental trajectories (slopes) of the two cohorts. The amounts of four variables differed between cohorts: Cohort 2 infants had less sleep-wake transition, more active sleep, less active sleep without REM, and more regular quiet sleep. These findings suggest that developmental patterns of sleep-wake states are stable enough in the preterm period that deviant individual patterns might be used to identify infants with neurological problems.