Introduction: Slovenian breast cancer screening programme, called DORA, is an organized popoulation based programme. It provides screening mammography every two years for all women in Slovenia aged 50 - 69 years old. In order to ensure the quality of mammograms in DORA programme, the mammograms of all radiographers are assessed once or twice a year. In Slovenia, we have our own evaluation system, which is a combination of PGMI criteria and Dokustufen. Purpose: To present the most common errors that occur in the CC and the MLO projections and also to investigate whether the quality control brings improvement of mammographic images. Methods: The methods of work comprise a literature review and the retrospective analysis. We collected data of 4928 mammograms that were made between 2011 and 2013. We used PGMI evaluation system. Results and discussion: We presented the results and comparisons in the tables and graphs for CC and MLO projections. We found that there were 22.08 % more errors on the MLO projections. The most common errors in the CC projection of both breasts were incorrect display of the nipple, skin folds, PNL, apperance of asymmetry and the apperance of inadequate display of the lateral parenchyma. The most common errors in the MLO projection of the both breasts were also skin fold, apperance of asymmetry and the incorrect display of the nipple. We also found that the most common erros were evaluated as level 2 errors. Conclusion: Between the years of 2011 and 2013, we did not notice any major increase or decrease of errors in mammograms. It is important to say, that Slovenia got its own evaluation system in the year of 2013 and also needed some time to educate all the radiographers working in screening programme DORA.