A 10-month longitudinal household study of pre-school children and their families was undertaken with monthly visits collecting epidemiological data and nasopharyngeal swabs in Hertfordshire, England from 2001 to 2002. Pneumococcal culture was with standard methods. In total, 121 families (489 individuals) took part. Mean prevalence of carriage ranged from 52% for age groups 0-2 years, 45% for 3-4 years, 21% for 5-17 years and 8% for >or=18 years. Carriage occurred more than once in 86% of children aged 0-2 years compared to 36% of those aged >or=18 years. The most prevalent serotypes in the 0-2 years age group were 6B followed by 19F, 23F, 6A and 14. Young children were responsible for the majority of introductions of new serotypes into a household. Erythromycin resistance (alone or in combination) occurred in 10% of samples and penicillin non-susceptibility in 3.7%. Overall the recently licensed 7-valent conjugate vaccine (PCV) would protect against 64% of serotypes with no intra-serogroup cross protection and 82% with such protection. Nasopharyngeal carriage of S. pneumoniae is common in a UK setting in the pre-conjugate vaccine era. PCV would protect against a large proportion of carriage isolates. However, the impact of vaccination on non-vaccine serotypes will need to be monitored.