PURPOSE: The aim of this study was to evaluate the optimal time for ROP screening and to find any preventive risk factors related to the progression to threshold ROP. METHODS: A retrospective analysis of ROP-diagnosed inborn neonates with gestational age of less than 32 weeks and birth weight of less than 1, 500 g who were admitted to Kangnam Sacred Heart Hospital from January 1997 to December 2003 were done. ROP infants were classified into two groups: PreT-ROP (prethreshold ROP, n=28) and T-ROP (threshold ROP, n=23). First ophthalmic examination time, time of onset ROP, time of threshold disease requiring cryotherapy or laser therapy, and risk factors were compared between two groups. RESULTS: Among all 220 infants, 51 (23.2%) patients were found to have ROP and 23 progressed to threshold ROP (10.5%). First ophthalmic examination time was significantly earlier in T-ROP for chronologic age. T-ROP was diagnosed at even 32+3 gestational weeks. Factors contributed to progression of ROP were gestational age, birth weight, Apgar score at 1 minute, duration of hospitalization, duration of TPN, duration of ventilator and oxygen supply, and number of transfusion. With logistic regression analysis, only birth weight (OR=1.001, 95% CI=1.001-1.016), duration of ventilation supply (OR=0.697, 95% CI=0.501-0.973), number of transfusions after the diagnosis of ROP (OR= 0.090, 95% CI=0.014-0.678) were relate to progression of ROP. CONCLUSION: For premature infants under 1, 500 g or 32 weeks and below should receive first ophthalmologic examination at least before 33 postconceptional age. The progression to threshold ROP after the diagnosis of ROP could be prevented by minimizing the duration of ventilator apply and number of transfusion after diagnosis of ROP.