Background.Meningococcal C conjugate (Men C) vaccines have been routinely used in the UK since November, 1999. Little information exists regarding antibody persistence or immunologic memory after infant vaccination or response to a first dose at 4 years.Methods.Ninety-five children immunized at 2, 3 and 4 months of age with 0 or 3 doses of Men C vaccine, boosted with Men C or meningococcal A/C polysaccharide vaccine at 12 months, received a single dose of Men C vaccine at 4 years; 103 age-matched controls were recruited. Pre- and postvaccination Men C IgG (enzyme-linked immunosorbent assay) antibody titers and serum bactericidal activity (SBA) were measured. Safety data were also collected.Results.Baseline SBA titers of ≥1/4 were observed in 87% of children after at least 3 doses of Men C vaccine in infancy compared with 21% of controls. Reciprocals of postvaccination SBA geometric mean titers in those with four prior doses [3803 (95% confidence interval 3489, 4146)] were significantly higher than controls [33 (95% confidence interval 20, 55)] (P< 0.001). Memory was attenuated by the 12-month meningococcal A/C polysaccharide booster [734 (95% confidence interval 484, 1115)] (P< 0.001). All children had IgG responses to a first dose of Men C vaccine, 80% achieving SBA titers of ≥1/4 (77% ≥1/8). The vaccine was safe and well-tolerated.Conclusion.Infant immunization with Men C produced persistent antibody and immunologic memory at 4 years. All children made IgG antibodies after a first dose at this age, with 80% showing bactericidal activity. Clarification of the best measures of Men C vaccine-induced protection is needed, through correlation of immunogenicity data such as this with UK vaccine efficacy estimates.