Background. Seroprevalence studies suggest that vaccination coverage of 90%-95% is needed to eliminate measles. In Bangladesh, routine measles vaccination coverage rates have recently reached 80%-85%. The Government of Bangladesh implemented catch-up vaccination through supplementary immunization activities (SIAs). The aim of the present study was to understand the impact of SIAs on immunization services and the health system. Methods. The study was conducted at 4 sites, all of which had relatively low routine vaccination coverage rates. A document review was performed, followed by interviews of key personnel selected by purposive and snowball sampling. A staff profiling survey was also undertaken. Results. Despite overall high levels of immunization, the expanded program on immunization for measles has not reached the coverage levels targeted by the Government of Bangladesh. The first SIAs vaccinated 35 million children, and the second targeted an additional 20 million. According to data and respondents, implementation of the SIAs was successful with sufficient funds being available, although timely disbursement of funds was inadequate in some areas. Staff were well motivated, and additional training helped boost a positive approach to vaccination. Conclusions. The SIAs had a positive impact on health and immunization systems and have created a framework on which other health care interventions for bacterial and viral diseases could be based.