Background: The COVID-19 pandemic caused a disruption in health services, specifically in cancer prevention, during quarantine. Argentina had one of the most extensive social isolation policies, which began on March 20, 2020. We analyzed the pandemic’s impact on the number of new patients and tumor stage distribution for those seen between March 20, 2020 and March 18, 2021 (Group B = Pandemic) and compared it with: i) those seen in the previous five years (Group A = Pre-pandemic); and ii) those seen between March 20, 2021 and December 31, 2022 (Group C = Post-pandemic). Methods: Our database comprised all new patients admitted to Henry Moore Cancer Institute (IOHM) in Buenos Aires, Argentina between March 20, 2015 and December 31, 2022. It included population characteristics at the time of first consultation, diagnosis and tumor stage according to TNM AJCC (8th Edition). The comparisons were made through Statistical t-Test and Chi-square homogeneity test. Results: Between March 20, 2020 and December 31, 2022, 17833 new patients were admitted to IOHM. The distribution of diagnoses was similar for all three groups: Breast (23%), Colorectal (11%), Prostate (10%), Gynecological (9%), Lung (7%), Renal (5%), Lymphomas (5%), Cervix (4%), Head and neck (3%), Pancreas (2%), Melanoma (2%), Sarcomas (2%), others (17%). Table 1 summarizes our main findings. The incidence of in situ tumors (stage 0) showed a significant reduction during the first year of COVID-19 in five tumors, despite strong evidence for screening benefits in all of them: Breast, Colorectal, Prostate, Melanoma and Cervix. The decrease in stage I new patients was observed in virtually all tumors, as well as the relative increase in stage IV (most notable in lung cancer, colorectal cancer and melanoma). Conclusions: A) During the first year of the COVID-19 pandemic, there was an expected quantitative decrease in new patients compared to the previous five years. B) However, we have also found qualitative changes in the characteristics of patients seeking a first consultation: younger patients, with worse ECOG, less early stages (0 and I) and more advanced stages (III and IV) in practically all tumors. C) These figures have improved in the 22 months following the lockdowns, but have not yet recovered to the values of pre-pandemic years. D) Given the strong evidence available for screening benefits in all of the five tumors that have seen a reduction in early stages consultation, preventative actions and campaigns must be reinforced.