Introduction: The WHO’s Global Advisory Group recommends an immunization strategy that offers or administers vaccines to all eligible children at every opportunity. This strategy has potential obstacles, namely Missed Opportunities of Vaccination (MOV) which are defined as all contacts with health services but children do not get the required vaccinations even though the child meets the requirements to be vaccinated. This research was aimed to analyze the prevalence of MOV and parental knowledge of immunization. Methods: Mothers who had children aged 9 months to 15 years of age participated, with a large sample of 1849 children. The research locations were 30 selected urban and rural areas in the province of East Java. This research found that there is a significant relationship between mothers with little knowledge about immunization and MOV. Results: : Children with mothers who have little knowledge about vaccination are 6.73 times more likely to experience MOV compared to mothers who have a lot of knowledge about vaccination. Conclusion: Promoting immunizations, especially when considering the benefits, is expected to reduce the incidence of MOV that lead to the enhancement of vaccination timeliness, improvement of health service delivery efficiency in general, and synergy the curative and preventive care services efforts in healthcare facilities.