Transfusion-associated bacterial contamination (TABC), probably the most frequent transfusion-transmitted infection, may induce serious adverse events. Systematic information and documentation on determinants are lacking.The BACTHEM Study is a French matched case-control study assessing TABC determinants. Included were cases of TABC reported in France in a 2-year period, as determined from uniform definitions. Information on recipient-, blood component-, and donor-related potential determinants was collected on site. ORs were estimated by conditional logistic regression.Of the 158 cases of suspected TABC reported, 41 that involved transfusion with 25 RBCs and 16 platelet concentrates were included. Gram-negative rods accounted for nearly half of the bacteria species involved and for all six deaths. In comparison with the risk of TABC for patients receiving RBCs for anemia, the risk was higher for patients receiving RBCs for pancytopenia (OR, 7.3; 95% CI, 1.3-41.0) and for those receiving platelets for thrombocytopenia (OR, 5.3; 95% CI, 1.2-24.1). Other potential determinants were platelet transfusion for pancytopenia (OR, 4.5; 95% CI, 0.5-40.0), immunosuppressive treatment (OR, 2.8; 95% CI, 0.7-10.6), shelf-life of more than 1 day for platelets or 8 days for RBCs (OR, 2.6; 95% CI, 0.7-9.6), and more than 20 previous donations by donors (OR, 1.9; 95% CI, 0.7-5.3).This first comparative study revealed TABC determinants that suggest approaches for prevention.