Reports of abscesses after the use of diphtheria and tetanus toxoids and pertussis vaccine (DTP vaccine) from two different lots (No. 1 and 2) of a single manufacturer (manufacturer A) prompted an investigation into the rates of abscess formation following the use of DTP vaccine from several different manufacturers. A total of 74 abscesses for lot 1, 16 for lot 2, and three for other DTP products was uncovered. The overall rate after lots 1 and 2 was 1.1 per 1,000 doses administered compared with 0.01 per 1,000 doses for DTP vaccine from other manufacturers (P less than .0001). Faulty technique, site and route of inoculation, microbiologic contamination, and hypersensitivity were ruled out as likely explanations for the increase in abscesses among recipients of DTP vaccine from manufacturer A. Use of a single needle to withdraw vaccine from the vial and to inoculate the vaccinees, combined with high aluminum adjuvant content in the implicated vaccine, may have led to an increased rate of abscess formation.