Background & Objectives : The length of stay (LOS) is a useful indicator that can be used according to the objective to improve hospital care performance. The purpose of our study was to find factors affecting infants LOS in NICU at Children's Medical Center, Imam Khomeini Hospital, Tehran University of Medical Sciences, using the Cox multiple hazards regression model.

  Methods : This historical cohort study reviewed 369 medical records of all NICU admitted newborns at Children's Medical Center in 2009. The required data were collected through a data collection form. The Cox multiple hazards regression model was used to determine the factors affecting LOS in infants who were discharged on the physician‘s order.

 Results: The median of stay in NICU was 9 days. Of 369 infants, 272 were discharged with improvement. The results of multiple Cox proportional hazards regression model showed the following factors were associated with LOS in the NICU: prematurity, referral from other hospitals, gastrointestinal diseases and infections, central venous catheterization, mechanical ventilation, and antibiotic therapy (P < 0.05).

  Conclusion : Cox proportional hazards regression model should be used when the dependent variable is time and we have censored data. Improving prenatal health care, constructing NICU in hospitals with high risk labor, reduction of preterm birth risk factors, and improving primary health-care services can help us to reduce LOS in NICU.