Transmission of HIV through transfusion occurred in a 78-year-old male after receiving red blood cells from a 27-year-old woman, who was negative for HIV antibodies and p24 antigen at the time of donation, but seroconverted thereafter. Plasma viral load at donation was 2538 HIV RNA copies/ml. Phylogenetic analyses demonstrated that viruses from both patients clustered tightly together, belonged to subtype B clade, and had no primary drug resistance mutations. This case highlights the residual risk of HIV transmission from donors in the window period, and supports the implementation of nucleic acid testing in blood banks.