Infants with ROP are studied. They received 0.25 mg or 0.5 mg of intravitreal ranibizumab to either 1 eye (unilateral cases) or both eyes (bilateral cases) with vascularly active ROP. Serum samples are collected 1 day before and 1 day, 3 days, 1 week after intravitreal ranibizumab. The serum concentrations of ranibizumab and VEGF are measured by enzyme-linked immunosorbent assay, and the changes of the serum VEGF levels are determined.