PURPOSE: To assess the motivation and barrier factors influencing participation of women in opportunistic breast cancer screening in Belgrade, Serbia, and to detect changes in these factors over time. METHODS: A cross-sectional study has been carried out at the Institute for Oncology and Radiology of Serbia in 2009 and 2016 among women aged 40 to 69 years from Belgrade who came at the Institute for opportunistic breast cancer screening. The demographic characteristics, data regarding breast exams practices, screening motivators and barriers and sources of information on breast cancer were collected by self-administered questionnaire. RESULTS: The questionnaire was completed by 478 women in 2009 and 453 in 2016, with increase in women reporting regular mammograms or at least one previous mammogram (from 30.1% to 58.6%, p=0.000). In 2009, the most frequent motivating factors were health maintenance (36%), friend's advice, TV, cancer in the family or fear of breast cancer; in 2016, advice from gynecologist (significant increase from 10.9% to 37.7%, p=0.000), health maintenance, family cancer and fear of cancer. The most frequent reasons for not going to exams regularly were absence of breast problems, crowded doctor's offices and no family breast cancer. CONCLUSIONS: These findings provide information on motivation and barrier factors that may influence women's decision to participate in opportunistic breast cancer screening. Those factors have changed over time and the role of physicians has increased significantly. Further exploration of motivating and barrier factors and the extent of their association with actual women's behavior would be helpful for the development of interventions to improve organized and opportunistic screening participation.