Landuse mapping is widely used for planning, forecasting and management purposes in the fields, which deal with the interactivity between mankind and the environment. The aim of this study is to monitor landuse changes in Istanbul and to assess the thematic accuracy on the geographical information system designed for the landuse of the city of Istanbul from the 1940s to 2000. Landuse has been assessed for the 1940s, 1968, 1988 and 2000 using satellite images, aerial photographs and maps. Since landuse detection has been done for all these evaluation years, it would have been a very large and complex task for any automatic classification method. Thematic accuracy has been determined by sampling because records in the geo-database. Differences between the landuses and thematic accuracies for the predefined years and those for the year 2000 have been determined. Thematic accuracies are given for each landuse type and a total accuracy has been determined for the whole study. The accuracy of the study was determined in two ways. The first one refers to the geometric accuracy of the spatial components of a geographical information system, which is designed for this study, while the second one refers to the thematic accuracy of the defined landuse patterns.