Background: Developing countries like India are presently developing new strategies to increase immunization coverage and reach more children with quality vaccine. Children in urban slums have high vulnerability to illnesses as outbreak of vaccine-preventable diseases are more common in these communities, owing to high population density and migratory population. Objective: To assess the immunization coverage and various sociodemographic factors affecting the same in urban slums of Lucknow. Material and Methods: A community-based cross-sectional study was conducted in slums of Lucknow city from August 2014 to July 2015. A total 327 children in age group 9 months to 2 years were enrolled in the study and their mothers (or primary caregivers) were interviewed through house-to-house survey with the help of a predesigned, pretested and semi-structured questionnaire. Results: About one-fourth (24.1%) of the children were completely immunized and 32.4% were partially immunized, while 43.4% were unimmunized. Significant association was found between mother’s occupation (p = 0.000), mother’s educational status (p = 0.013), and father’s educational status (p = 0.023) with immunization status of children. Conclusion: In this study, overall coverage of immunization was suboptimal in slums. So, there is an urgent need to review the current strategies of immunization with special focus in urban slums.