The reported incidence of measles in children of secondary school age rose in 1992, after a progressive decline between 1988 and 1991. This rise was maintained in 1993. Several school and community based outbreaks of measles have occurred in the United Kingdom. This paper reports the investigation of an outbreak of measles based in a secondary school, which took place in 1992. Thirty clinical cases were detected among the school's 840 pupils and 10 sporadic cases occurred outside the school. Twenty-one of the school cases provided samples of serum, in 19 of which measles IgM was detected. The overall attack rate was 3.6%, with no significant differences attributable to age and sex. Vaccine efficacy was about 90%. This outbreak is one of the first to be described in the United Kingdom, although other countries (notably the United States) have reported measles in teenagers. The small degree of spread in the community may reflect the current high uptake of measles, mumps, and rubella vaccine and the catch up campaign that took place in 1988. The feasibility and cost effectiveness of various policy options to prevent future outbreaks in secondary schools are now being evaluated.