The Coronavirus Disease 2019 (COVID-19) is caused by the new type of coronavirus, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), emerged in China at the end of 2019, and spread all over the world in a very short time. The World Health Organization (WHO) declared COVID-19 a global pandemic due to its rapid spread and lack of effective vaccination, immunization and treatment. In this study, we aimed to make a retrospective evaluation of forty-five days of COVID-19 cases and deaths simulated by the modified mathematical model which was previously established and published by us. 30 days and 45 days of COVID-19 cases and deaths which were estimated by our modified mathematical model were compared to the real cases and deaths in Turkey since 15.03.2020. The COVID-19 cases increased to a significant level on 20.03.2020 in Turkey. The modified mathematical modeling results’ estimation accuracy remained above 90% until 13.04.2020. This rate was 78.40% on 28.04.2020. The modified mathematical modeling estimation for the COVID-19 deaths started on 20.03.2020 because the number of deaths were then significant enough for estimation with the modified mathematical model. The estimation accuracy for the number of deaths was 100% with a value of 37 on 23 March 2020, and after that, it had always remained above 90% until 13.04.2020. Finally, it was 66.08% on 28.04 2020. The estimation accuracies of the modified mathematical model about the COVID-19 cases in Turkey for 30 days and 45 days were above 90% and 78% respectively. The estimation accuracies of the modified mathematical model about the COVID-19 deaths in Turkey for the 30-day and 45-day periods were above 90% and 66%, respectively. This result suggests that the modified mathematical model is available for estimating the course of disease outbreaks and pandemics. The model should be developed through future studies, which will improve its estimation accuracy.