Two fractions of a three-day-old apheresis platelet collection from a known habitual donor were transfused to two children with thrombocytopenia and bleeding. Both patients developed evidence of severe infection during the transfusion. One died despite intensive care and antimicrobial therapy. The other, whose transfusion was cut short, recovered. A Klebsiella oxytoca strain was recovered from the two transfusion bags, from a third unused bag, and from blood samples from the patient who died. Genotyping results established that all these isolates were identical. Tests for K. oxytoca were negative on the batches of blood donation material, the bottle of antiseptic used, and throat and stool specimens from the donor and phlebotomists. The most likely hypothesis is that the donor developed transient asymptomatic bacteremia during the 136-minute-long collection procedure and that the organism subsequently grew in the platelet collections, which were kept at 20-24 degrees C with agitation for three days before being used.