BACKGROUND: The inclusion of 7-valent pneumococcal conjugate vaccine (PCV7) into national immunization programs in many countries has significantly decreased the incidence of disease caused by Streptococcus pneumoniae. However, a substantial portion of disease remained and, in some areas, there has been an increase in disease produced by serotypes not included in PCV7. A 13-valent pneumococcal conjugate vaccine (PCV13) was studied in healthy Brazilian infants in a phase 3, double-blind, randomized study. METHODS: Infants were randomized to receive either PCV7 or PCV13 at 2, 4, 6, (doses 1-3), and 12 (toddler dose) months of age, along with routine pediatric vaccinations (diphtheria, tetanus, whole-cell pertussis, and Haemophilus influenzae type b vaccine). Pneumococcal anticapsular polysaccharide-binding immunoglobulin G (IgG) responses and antibody responses to pertussis antigens were measured 1 month after both dose 3 of the infant series and the toddler dose. Safety and tolerability were also assessed. RESULTS: The proportion of subjects achieving a serotype-specific IgG concentration ≥0.35µg/mL measured 1 month after the infant series was comparable in the PCV13 (≥94.2%) and PCV7 (≥93.0%) groups for the 7 serotypes common to both vaccines. The percentage of responders for the 6 additional serotypes ranged from 87.1 to 100% for PCV13. The percentage of responders varied across the pertussis antigens studied, but was not different in PCV13 and PCV7 recipients. Overall, the safety profile of PCV13 was comparable with that of PCV7. CONCLUSIONS: PCV13 was comparable to PCV7 in safety and tolerability, elicited comparable immune responses to the common serotypes, and did not interfere with immune responses to concomitantly administered whole-cell pertussis vaccine. The robust immunogenicity exhibited by PCV13 for the additional serotypes suggests that it could provide significant protection against these serotypes.