PURPOSE: The ophthalmologic screening examination in extremely low birth weight (ELBW) infants shoud be done at a postconceptional age (PCA) of 31 to 33 weeks or the chronological age (CA) of 4 to 6 weeks. If the first ophthalmologic examination in ELBW infants is perfomed at 31 to 33 weeks PCA, there is a risk of threshold retinopathy of prematurity (ROP) having already developed on the first examination. The risk of a visual loss is high if threshold ROP has already developed before the initial screening examination of ROP. Therefore, we investigated the ideal timing of the initial ophthalmologic screening examination based on PCA and CA in ELBW infants. SUBJECTS: The medical records of 38 ELBW infants (<1000 g at birth) admitted to neonatal intensive care unit of Severance hospital between January 1991 and December 2000, whose follow-up ophthalmologic examinations were available, were reviewed retrospectively. We investigated the PCA and CA at the diagnosis of prethreshold ROP and threshold ROP. RESULTS: Sixty-five percent of subjects was diagnosed with prethreshold ROP and 64% of the infants progressed to threshold ROP. The median time of progression from prethreshold ROP to threshold ROP was 14 days (2-33). Twenty-four percent was diagnosed with prethreshold ROP on the first eye examination. Prethreshold ROP was diagnosed as early as 33 weeks PCA and threshold ROP was diagnosed as early as 35 weeks PCA. Prethreshold ROP was diagnosed as early as 4 weeks CA and threshold ROP was diagnosed as early as 6 weeks CA. CONCLUSION: We suggest that the initial screening examination for ROP should be performed by CA or PCA in ELBW infants, whichever is earlier, to detect prethreshold ROP before its progression to threshold ROP.