Immunization with high-titer measles vaccines has been associated with excess mortality in children 2–4 years after vaccination. In this study, immunologic parameters in 64 Peruvian children who had been immunized an average of 27 months earlier with high-titer vaccines were compared with parameters in 76 recipients of low-titer vaccines. Delayed-type hypersensitivity, lymphocyte phenotype distributions by flow cytometry, and lymphoproliferation after phytohemagglutinin (PHA) stimulation were assessed. High-titer recipients had smaller indurations to tetanus, diphtheria, and Proteus (P <.05) antigens, decreased PHA stimulation (P =.04), and a lower percentage of CD4+ lymphocytes (P =.04) than low-titer recipients. After adjustment for sex, concurrent illnesses, and other variables in regression analyses, high-titer recipients had a lower percentage of CD4+ lymphocytes (P =.025) and decreased lymphocyte proliferation to PHA (P =.058). These results may provide a clue to the pathogenesis of delayed excess mortality after high-titer measles vaccination in some developing countries. © 1993 by The University of Chicago.