A major purpose of a state-wide survey to document the vaccination status of 1,003 2-year-old children was to identify factors associated with failure to receive the recommended vaccinations. With a basic series of immunization defined as three doses of diphtheria-tetanus-pertussis (DTP), three oral polio vaccine (OPV), one measles, and one rubella, 72.5% of the children had completed the series. When the completed series was redefined to include a fourth DTP and mumps vaccine the rate of completion dropped to 40.8%. However, 59.1% of the children who had not completed this optimal series could be brought up-to-date with a single visit to their provider of medical care. Demographic variables independently associated with completion of the basic series were increased paternal education (P less than .001), increased maternal education (P less than .02), smaller family size (P less than .01) and higher socioeconomic status, as determined by census tract or rural town of residence (P less than .02). Race was not found to be a factor associated with vaccination rates when socioeconomic status was controlled. Patients who received their vaccinations from private physicians had a better vaccination rate than those who attended health department clinics. This difference persisted even when socioeconomic status was controlled by residence (P less than .02). The simultaneous comparison of parental education and family size demonstrated that a child having one parent with less than 12 years education or having at least three siblings has a fourfold greater risk of failure to complete his immunization than children whose parents are both college graduates. By using paternal and maternal education level and family size as screening variables, children at high risk for failure to complete their immunizations could be identified prospectively and made the target of intervention programs to improve compliance.