The UK was the first place to introduce meningococcal serogroup C conjugate (MCC) vaccines. From November, 1999, all people younger than 18 years, about 14 million individuals, were offered MCC immunisation. The uptake rate was more than 70% by November, 2000. We compared the carriage of meningococci in isolates we obtained from 14,064 students aged 15-17 years during vaccination in 1999, with those from 16,583 students of the same age surveyed 1 year later. Carriage of serogroup C meningococci was reduced by 66% (p=0.004). Our results show that MCC vaccines protect against carriage of meningococci that express serogroup C polysaccharide capsules.