Although it is possible that relative tissue hypoxia caused both EPO stimulation and risk of ROP, and that transfusion is merely a marker of low hemoglobin, the correlation between transfusion and risk of ROP remained significant after adjusting for [EPO] (r = 0.6, p = 0.014 at 1 week; r = 0.8, p < 0.001 for AUC[EPO]0–2 weeks], S4 Table) or hemoglobin (r = 0.7, p = 0.002 at 1 week, r = 0.8, p < 0.001 at 2 weeks, S4 Table), which suggests the impact of transfusion on ROP is not solely driven by lower oxygen carrying capacity. Here, EPO is linked to retinopathy of prematurity.