The World Health Organization (WHO) launched the Expanded Programme on Immunization (EPI) more than 10 years ago with the aim to protect the world's children against diphtheria, pertussis, tetanus, polio, measles and tuberculosis before their first birthday. In many developing countries, national EPIs have achieved high immunization coverage rates. However, decreasing health budgets threaten to affect the success of the programme. With limited resources, improving EPI service quality becomes one of the most important options to sustain or increase immunization coverage. This case study from Zimbabwe presents a new model to assess and improve EPI service quality at district level. The model is based on an iterative and participatory action research strategy, using a combination of quantitative and qualitative reserach methods. This approach bears relevance to quality improvement in other health programmes as well.