The loss of red cell alloantibodies over time was analyzed in 160 patients with 209 antibodies retested 1 to 60 months after initial identification. The mean follow-up consisted of 3.6 specimens taken over 18 months. Twenty-nine percent of clinically significant and 72 percent of clinically insignificant antibodies were not detected on at least one follow-up screening. Anti-Jka and anti-C were lost in 59 and 45 percent of cases, respectively. No significant differences in overall antibody loss were found according to sex, diagnosis, or the presence of multiple antibodies. Patients under 20 years of age may be more likely to lose significant antibodies. Pretransfusion review of previous records is vital for the prevention of delayed hemolytic transfusion reactions, because of the high number of clinically significant antibodies that are undetected in subsequent routine screening.