The study investigated the levels of vascular endothelial growth factor-A (VEGF-A) and free ranibizumab in serum and breast milk of lactating women after intravitreal injection of ranibizumab. A thirty-year-old woman breastfeeding a six-month-old son was treated for branch retinal vein occlusion (BRVO) of the right eye. Serum and milk samples were collected before and up to 4 weeks after injection and analyzed by enzyme-linked immunosorbent assay (ELISA). Both serum and milk ranibizumab concentrations increased rapidly on the first day, peaking at 15.35 ng/ml in serum and 3.14 ng/ml in milk. The serum VEGF-A level decreased from 29.49 ng/ml to 12.75 ng/ml in the first three days and then slowly returned to the original level. VEGF-A levels varied in breast milk, but serum levels remained stable. Systemic exposures to ranibizumab have been observed in a lactating patient. Further studies are needed to assess the safety of anti-VEGF therapy for infants.