Objective To examine missed opportunities to administer an eligible vaccination (MOs) and their contribution to underimmunization in contemporary pediatric practices. Study design This study was a retrospective analysis from 42 diverse pediatric practices located throughout the US. Medical records of 50 randomly selected children 3-18 months of age per practice were reviewed in Spring 2013. Immunization status for age and MOs were assessed as of each encounter and as of March 1, 2013. Results Of 2076 eligible patients, 72.7% (95% CI 67.6-77.9) were up-to-date with receipt of standard vaccines. Most children (82.4%; 95% CI 78.3-85.9) had at least 1 MO, and 37.8% (95% CI 30.0-46.2) had at least one MO to administer an overdue vaccination. After adjustment, risk of underimmunization was 3.5 times greater for patients who had ever experienced an MO for an overdue vaccination compared with those who had not (adjusted relative risk = 3.5; 95% CI 2.8-4.3). If all age-appropriate vaccinations had been administered at the last recorded encounter, 45.5% (95% CI 36.8-54.5) of the underimmunized patients would have been up to date at the time of assessment. Conclusion MOs were common and contributed substantially to underimmunization in this contemporary sample of diverse primary care practice settings.