Purpose: The purpose of this study was to investigate the sleep/activity patterns of premature babies. Method: The subjects were 55 premature babies who were hospitalized in the Neonatal Intensive Care Unit (NICU) of one University Hospital in Daejeon from September 1, 2004 to March 31, 2006. The sleep/activity patterns of the premature babies were observed for 24 hours a day for 7 days. The observations were made and recorded by the researcher and 5 trained research assistants using NCASA records developed by Barnard et al. (1979). For statistical analysis, frequency, average, t-test, and Pearson correlation coefficients were used. Results: The infants slept more and had less awake periods during daytime than full-term infants. As the hospital stay of the premature babies increased, the amount of daytime sleep and total daily sleep decreased, and the amount of daytime awake periods increased. Conclusions: The findings suggest that the level of physical and physiological maturity of premature babies is one of the major variables that affect their sleep/activity patterns. Furthermore, the findings of this study will provide information about the sleep/activity patterns of premature babies, and present baseline data to develop the nursing intervention programs which support optimum development for the premature babies.