Introduction: Breast cancer is the most commonly diagnosed cancer in women. In Croatia, in 2022, according to the International Classification of Diseases (ICD-10), there were 3,108 cases of malignant breast neoplasms in women, with an incidence rate of 145.60 per 100,000 inhabitants. On June 29, 2006, the Government of the Republic of Croatia adopted the National Early Detection Program for Breast Cancer. The aim of this program is to ensure early detection of breast cancer, reduce mortality from breast cancer by 25% within five years from the beginning of its implementation, and improve the quality of life for women suffering from breast cancer. The implementation of the National Early Detection Program (NPP) for breast cancer involves regular mammograms for women aged 50 to 69. Women in this age group are invited to free mammograms every two years through official invitations sent to their home address. Objectives: The objectives of this study are to determine the participation rate of women in the mammogram screening program within the National Early Detection Program at the Clinical Hospital Center Rijeka (KBC) and to identify whether there are variations in participation rates related to the seasons. A secondary objective is to identify the age group with the highest participation in the mammogram screening as part of the National Early Detection Program. Materials and Methods: This retrospective study included women aged 50 to 69 who were invited to undergo mammography screening as part of the National Breast Cancer Prevention Program at the Department of Abdominal Radiology and Breast Radiology of the Clinical Institute for Diagnostic and Interventional Radiology of the Clinical Hospital Center in Rijeka in the period from 1 January 2021 to 31 December 2023 during the implementation of the 6th and 7th cycles of the National Breast Cancer Prevention Program. The data required for the study were collected in the hospital information system (IBIS) and the radiology information system (ISSA). The Z-test for proportions and the Chi-square test were used to test the hypotheses. All hypotheses were tested at a statistical significance level of p < 0.05. Results: The proportion of women who responded to the invitation for a mammogram screening as part of the national screening program during the study period was 42.79%. The most represented age group was women aged 65 to 69. The highest participation rate was observed in February. Conclusion: The proportion of women who responded to the invitation for a mammogram screening as part of the national screening program is significantly below the acceptable level of 70%, and even further from the desired level of over 75%. The most represented age group was women aged 65 to 69, which is expected since this population is likely retired. The analysis of monthly participation shows that February had the highest participation. Comparing these data with screening effectiveness indicators, it is clear that there are significant deviations that point to the need for improved strategies for raising awareness and motivating women to participate in the program. Special attention should be given to increasing participation in order to approach the acceptable and desirable standards set by the Croatian Guidelines for Quality Assurance in Breast Cancer Screening and Diagnosis, which is crucial for achieving better effectiveness in early breast cancer detection.