A recent resurgence in serious infections due to Haemophilus influenzae type b (Hib) has been observed in the United Kingdom. More information on Hib transmission in the population is required in order to better understand the mechanism of this increase. The Public Health Laboratory Service (subsumed into the Health Protection Agency since April 2004) conducted four cross-sectional studies of asymptomatic oropharyngeal Hib carriage in children attending day-care nurseries in England and Wales in 1992, 1994, 1997 and 2002. These demonstrated a marked reduction in the prevalence of Hib colonization over time since vaccine introduction (3.98% in 1992; 0.70% in 1994; 0% in 1997; 0% in 2002), which did not explain the increase in invasive disease reports from 1999 onwards. We believe that a reduction in antibody levels over the first 5 years of life in immunized children in recent years has fuelled the rise in reported cases in the absence of an obvious increase in transmission.