Since 1999, the number of cases of Haemophilus influenzae type b (Hib) disease in the UK has risen. We investigated the role of population immunity in this change by testing more than 2600 serum samples from children aged 1-15 years. After the introduction of the routine Hib conjugate vaccination programme for infants, median antibody titres rose significantly in 1-year-olds. Individuals who received their first dose of vaccine at age 1-4 years in the original catch-up campaign initially had much higher concentrations of antibody than those who had been immunised in infancy. A second catch-up campaign in children aged 6 months to 4 years should be highly effective in boosting immunity and reducing disease in the short term.