Objective To assess the role of maternal attitudes and other factors associated with infant vaccination status. Study design Data on reported vaccination status were analyzed from a nationally representative prospective survey of mothers of 2- to 6-month-old infants. Weighted univariate and multiple logistic regression analyses were conducted. Latent profile analysis of mothers reporting nonimmunized infants identified distinct groups, Results Of 3268 mothers, 2820 (weighted 86.2%), 311 (9.1%), and 137 (4.7%), respectively, reported their infant had received all, some, or no recommended vaccinations for age. Younger infants and infants with younger mothers were more likely to have received no vaccinations. Mothers with neutral and negative attitudes toward vaccination were >3 (aOR 3.66, 95% CI 1.80-7.46) and 43 times (aOR 43.23, 95% CI 20.28-92.16), respectively, more likely than mothers with positive attitudes to report their infants had received no vaccinations. Two subgroups of mothers reporting that their infants had received no vaccinations were identified: group A (52.5%) had less than positive attitudes and less than positive subjective norms about vaccination (ie, perceived social pressure from others); group B (47.5%) had positive attitudes and positive subjective norms. Group A mothers were more likely to be white (76.1% vs 48.3%, P =.002), more educated (43.5% vs 35.4% college or higher, P =.02), and to exclusively breastfeed (74.9% vs. 27.3%, P <.001). Conclusions Although access barriers can result in nonvaccination, less than positive maternal attitude toward vaccination was the strongest predictor. Strategies to improve vaccination rates must focus on both improved access and better understanding of factors underlying maternal attitudes.