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To further understand state development of preterm infants throughout hospitalization and the effects of selected infant characteristics on state development.Secondary data analysis of a 2-group, experimental design study.Two nurseries in a Northwest medical center.Ninety-seven hospitalized, medically stable, preterm infants. Fifty-one subjects were females.Two hundred and eighty-five real-time video recordings of infants performed during 4 hour interfeeding intervals. Sleep-wake states were coded at 15 second intervals.Active sleep was the dominant state across postmenstrual ages. Although not statistically significant, preterm infants showed developmental changes in state organization with increased quiet sleep, drowsy, and awake, decreased active sleep, and more defined and less diffuse states over age. A significant gender effect was found, with males having less active sleep (p=.012), more drowsy (p=.03), more awake (p=.043), less defined (p=.002), and more diffuse (p=.001) states compared with females.The predominance of active sleep during the preterm period reflects level of brain maturation. The results emphasize individual variations in state organization influenced by endogenous and environmental factors. Gender differences are potential sources of individual variation.