The effect of the age of RBCs on anti-RBC alloimmunization has not been investigated in previous studies of the RBC storage lesion. D-negative recipients of at least 1 D-positive RBC unit were identified. Responders produced an anti-D, while nonresponders had not made anti-D when the database was searched. The 2 groups were matched for age, sex, length of serologic follow-up, and hospital location. There were 29 responders and 58 nonresponders. The median number of all RBCs transfused to the responders was 6, vs 10 for the nonresponders (P < .01); the median age of the RBCs was 15 vs 14 days, respectively (P = .70). Responders received a median of 3 D-positive RBC units vs 4 D-positive RBCs for nonresponders (P = .02); median ages of the D-positive RBCs were 16 vs 14 days, respectively (P = .21). There was no association between the age of transfused RBCs and the likelihood of anti-D alloimmunization.