The incidence of invasive meningococcal disease is highest in infants. A quadrivalent meningococcal serogroups A, C, W-135 and Y tetanus toxoid conjugate vaccine (MenACWY-TT) was evaluated in children 9-12 months of age. We randomized infants (1:1) to receive 1 dose of MenACWY-TT at 12 months of age (ACWY-1 group) or 2 doses at 9 and 12 months (ACWY-2). We measured immunogenicity after each dose and 1 year after completing vaccination using human serum bactericidal antibody (hSBA) assays according to prespecified criteria of ? 1:8. Local and general symptoms were solicited for 8 days after vaccination. Adverse events were recorded for 6 months after the last dose. We enrolled and vaccinated 349 subjects, of whom 248 reenrolled at Year 1 for evaluation of antibody persistence. Percentages of subjects with postvaccination hSBA ? 1:8 in the ACWY-1 group were 79.5%, 94.6%, 50.8% and 56.1% and in the 2-dose group (ACWY-2) were 88.4%, 100%, 99.3% and 99.3% postdose 2 for serogroups A, C, W-135 and Y, respectively. At Year 1, 80.0-99.1% in each group had hSBA ? 1:8, except for serogroup A, for which 20.6% (ACWY-1) and 25.9% (ACWY-2) retained hSBA ?1:8. Both schedules were well-tolerated, with no observed increase in reactogenicity after the second dose. MenACWY-TT was immunogenic when administered as a single dose at 12 months of age, or as 2 doses at 9 and 12 months, and had a clinically acceptable safety profile. Good antibody persistence was observed through 12 months postvaccination after both treatment schedules for serogroups C, W-135, Y.