The study assessed the impact of an EPI (Expanded Programme on Immunization) intervention package, implemented within the existing service-delivery system, to improve the child immunization coverage in urban slums of Dhaka, Bangladesh. This intervention trial used a pre- and post-test design. An intervention package was tested from September 2006 to August 2007 in two urban slums. The intervention package included: (a) an extended EPI service schedule; (b) training for service providers on valid doses and management of side-effects; (c) a screening tool to identify immunization needs among clinic attendants; and (d) an EPI support group for social mobilization. Data were obtained from random sample surveys, service statistics and qualitative interviews. Analysis of quantitative data was based on a 'before and after' assessment of selected immunization-coverage indicators. Qualitative data were analysed using content analysis. Ninety-nine per cent of the children were fully immunized after implementation of the interventions compared with only 43% before implementation. Antigen-wise coverage after implementation was also significantly higher compared with before implementation. Only 1% drop-out was observed after implementation of the interventions while it was 33% before implementation. At baseline, a significantly higher proportion of children of non-working mothers (75%) were fully immunized compared with children of working mothers (14%). Although the proportion of fully immunized children of both non-working and working mothers was significantly higher at endline, fully immunized children of working mothers dramatically improved at endline (99%) compared with baseline (14%). The findings suggest the effectiveness of a 'package of interventions' in improving child immunization coverage in urban slums. However, further research is needed to fully assess the effectiveness of the package, to assess the individual components in order to identify those that make the biggest contribution to coverage, and to assess the sustainability of this package within the existing service delivery system, particularly on a wider scale.