A 9-day-old child developed a transfusion-transmitted hepatitis C virus (HCV) infection following a pre-seroconversion window-phase donation. Retrospective analysis of donor plasma revealed detectable HCV core antigen (154 fmol/l), as well as HCV RNA (87,000 IU/ml). Of 5.24 million Swedish plasma samples from December 1998 to September 2012, 5 additional window-phase donations were identified.