Objectives: To examine predictors of partial and full measles, mumps and rubella (MMR) vaccination catch-up between 3 and 5 years. Design: Secondary data analysis of the nationally representative Millennium Cohort Study (MCS). Setting: Children born in the UK, 2000-2002. Participants: 751 MCS children who were unimmunised against MMR at age 3, with immunisation information at age 5. Main outcome measures: Catch-up status: unimmunised (received no MMR), partial catch-up (received one MMR) or full catch-up (received two MMRs). Results: At age 5, 60.3% (n=440) children remained unvaccinated, 16.1% (n=127) had partially and 23.6% (n=184) had fully caught-up. Children from families who did not speak English at home were five times as likely to partially catch-up than children living in homes where only English was spoken (risk ratio 4.68 (95% CI 3.63 to 6.03)). Full catch-up was also significantly more likely in those did not speak English at home (adjusted risk ratio 1.90 (1.08 to 3.32)). In addition, those from Pakistan/Bangladesh (2.40 (1.38 to 4.18)) or 'other' ethnicities (such as Chinese) (1.88 (1.08 to 3.29)) were more likely to fully catch-up than White British. Those living in socially rented (1.86 (1.34 to 2.56)) or 'Other' (2.52 (1.23 to 5.18)) accommodations were more likely to fully catch-up than home owners, and families were more likely to catch-up if they lived outside London (1.95 (1.32 to 2.89)). Full catch-up was less likely if parents reported medical reasons (0.43 (0.25 to 0.74)), a conscious decision (0.33 (0.23 to 0.48)), or 'other' reasons (0.46 (0.29 to 0.73)) for not immunising at age 3 (compared with 'practical' reasons). Conclusions: Parents who partially or fully catch-up with MMR experience practical barriers and tend to come from disadvantaged or ethnic minority groups. Families who continue to reject MMR tend to have more advantaged backgrounds and make a conscious decision to not immunise early on. Health professionals should consider these findings in light of the characteristics of their local populations.