Associations between maternal age and preschool immunization coverage are unclear. This study aimed to determine if maternal age is associated with preschool immunization coverage and the importance of maternal age compared with other factors affecting vaccination coverage. Data from the 2001-2003 National Immunization Survey (NIS) were used to estimate vaccine coverage. Children were considered up-to-date (UTD) if they received > or =4 doses of DTaP, > or =3 doses of polio, > or =1 doses of MMR, > or =3 doses of Hib and > or =3 doses of Hep B. Bivariate and multivariate relationships between UTD coverage and maternal, child and household factors were evaluated. Classification tree analysis assessed complex interactions between maternal, child and household factors associated with UTD coverage and isolated the most important factors in predicting UTD coverage. UTD coverage was significantly associated with maternal age: coverage increased as maternal age increased. Coverage among children with 17 year old mothers was 64%; coverage among children of mothers 17-26 years old increased by 16.3% overall (approximately 1.8% per year). After 26 years of age, coverage did not increase significantly as maternal age increased. The relationship between maternal age and UTD coverage remained statistically significant after adjusting for a broad range of maternal, child and household factors. Classification tree analysis suggested that maternal age is the most important factor associated with vaccine coverage. More research is needed to determine the reasons for underimmunization of children born to young mothers.