A two dose schedule for measles, mumps, and rubella (MMR) vaccination was introduced in England and Wales on 1 October 1996 in order to prevent the accumulation of susceptible individuals and thus prevent future epidemics. A routine second dose of MMR vaccine is now offered with the diphtheria, tetanus and polio (DTPol) preschool booster. In addition a second dose of MMR is being offered to all children born since 1 January 1990 without documented histories of two doses of measles containing vaccine in a catch up programme due to end in March 1998. Progress in the catch up programme in Wales was assessed by examining uptake of all possible combinations of measles containing vaccines in four birth cohorts by 1 May 1997. Of 126,550 children born between January 1990 and March 1993 (aged 4 years, 1 month to 7 years, 3 months) and living in Wales on 1 May 1997, 63,855 (50.5%) had received two doses of measles containing vaccine and 120,668 (95.4%) had received at least one dose, but 5882 children (4.6%) had no record of having received any measles containing vaccine. Uptake of the preschool DTPol booster in the same cohort was 87.2%. Uptake of two measles containing vaccines varied between health authority (range: 35.4-58.3%) and between birth cohorts (range: 40.0-81.8%). Few (18.4%) previously unimmunised children had attended for the MMR catch-up dose. Successful elimination of measles may depend on targeting this group in order to improve coverage. Although this analysis was carried out during the catch-up programme, these data suggest that the programme may not reach the children who most need to be immunised.