The quality of vaccination services provided for children aged 12-23 months was determined in different districts of Istanbul, Turkey. Quality was assessed through a lot quality assurance sampling method, using data extracted from interviews conducted in 2001 with the mothers or carers of the children. Concurrently, geographic information system technology was used for integrated evaluation of the data from different lots and for presentation of the results as thematic maps. The results of this study helped to identify districts of the city in which vaccination services were below the expected level of quality and where an unacceptable number of children were not being fully vaccinated. Subsequent to this study, actions were taken by the agencies responsible to improve the coverage and quality of the vaccination services in these districts of Istanbul.