OBJECTIVE: Apnea of prematurity is one of the most common diagnoses in the NICU. Because resolution of apnea is a usual precondition for discharge from the hospital, different monitoring practices might affect length of stay for premature infants. Our objective was to compare the proportion of 33 to 34 weeks’ gestational age infants diagnosed with apnea in different NICUs and to assess whether variability in length of stay would be affected by the rate of documented apnea. METHODS: This was a prospective cohort study of moderately preterm infants who survived to discharge in 10 NICUs in Massachusetts and California. RESULTS: The study population comprised 536 infants born between 33 and 34/7 weeks of which 264 (49%) were diagnosed with apnea. The mean postmenstrual age at discharge was higher in infants diagnosed with apnea compared with those without apnea (36.4 +/- 1.3 vs 35.7 +/- 0.8; P < .001, analysis of variance). Significant inter-NICU variation existed in the proportion of infants diagnosed with apnea (range: 24%-76%; P < .001). Postmenstrual age at discharge also varied between NICUs (range: 35.5 +/- 0.6 to 36.7 +/- 1.5 weeks; P < .001). As much as 28% of the variability in postmenstrual age at discharge between NICUs could be explained by the variability in the proportion of infants diagnosed with apnea. CONCLUSIONS: NICUs vary in the proportion of moderately preterm infants diagnosed with apnea, which significantly affects length of stay. Standardization of monitoring practices and definition of clinically significant cardiorespiratory events could have a significant impact on reducing the length of stay in moderately preterm infants.