OBJECTIVE 1) To examine a wide range of potential risk factors for delayed childhood immunization against measles, mumps, and rubella and 2) to determine the parental response to proposed solutions regarding the problem of delayed childhood immunization. METHODS A case-control study in which subjects had (controls) or had not (cases) received measles, mumps, and rubella vaccine (MMR) by 2 years of age. Parents of subjects were also surveyed to find out whether they thought selected proposals to improve immunization rates would be successful. RESULTS Maternal education of high school or less at the time of the child's birth, more than or equal to two moves between birth and age 2, maternal age < 21 years of age at the time of the child's birth, more than or equal to two older siblings, participation in The Special Supplemental Food Program for Women, Infants, and Children (WIC), participation in the federal food stamp program, and incorrect knowledge of the recommended age for MMR significantly elevated the odds ratio (OR) for delayed immunization. At least two older siblings (OR = 3.2), maternal age < 21 years of age at the time of the child's birth (OR = 2.8), and incorrect knowledge of the recommended age for MMR (OR = 2.7) remained significant risk factors in a multivariate logistic regression model. Insurance status and cost factors were not significant risk factors for delayed immunization. Parents though that reminders for immunization and a central record system would have made obtaining immunizations easier. CONCLUSIONS Based on our findings of the importance of immunization knowledge and demographic characteristics as risk factors for delayed immunization, we suggest that a message (the recommended age for immunizations) and a target group for that message (families who move frequently, have older children, and are headed by younger parents) be evaluated as an intervention to improve immunization rates.