DNA image analysis is presently performed in many laboratories. Before general extrapolation of results between different laboratories, validation has to be performed for interlaboratory studies on DNA image analysis. The aim of this study was to establish the performance of different DNA image analysis instruments when measuring different diploid cells. On three separate parts of the same object slide, human liver cells, white blood cells, and imprints of a breast fibroadenoma were sampled. In this quality assurance interlaboratory study, 13 laboratories participated voluntarily. Two slides were sent to each participating laboratory: one Feulgen and one unstained to be stained according the participating laboratory in-house procedure. The features integrated optical density (IOD) and object AREA were recorded for each nucleus. For calculation of the results, the average IOD value of liver diploid cells was set at 2c. A striking difference was observed between the different presumed diploid cell types, from almost 1c to 3c. This variation was not dependent on central or in-house staining. Although in-house calibration was performed for each image analysis system, a surprisingly large variation existed in the reported object AREA, irrespective of the cell type. These results clearly demonstrate that measurements performed in one laboratory cannot be extrapolated to the outcome of others and support the need for standardization. The use of external control cells works well for comparison of instruments. In conclusion, in DNA image analysis quality control is necessary, standardization is obligatory, and the use of an internal control for determination of the diploid peak in a histogram of patient samples is recommended.