Background: Solid cancer is an independent prognostic factor for COVID-19 related mortality. Adverse prognostic factors in these patients include low performance status, lung cancer, advanced cancer stage and recent diagnosis. In this study, we further evaluated prognostic effects of cancer diagnosis and treatment variables and characterized changes in anticancer treatment plans due to COVID-19 diagnosis in a nation-wide cohort study. Methods: Patients with a history of solid cancer hospitalized with COVID-19 between March 2020 and February 2021 in one of the 20 participating institutions in Belgium were included. Patient demographics, comorbidities, COVID-19 hospitalization course and treatment, cancer and anticancer treatment characteristics, treatment changes due to COVID-19 and clinical outcomes in-hospital and during follow-up were retrospectively registered in a central database. The primary objective was to evaluate potential differences in 30-day and 3-month COVID-19-related mortality according to cancer and anticancer treatment characteristics. Results: A total of 946 patients (median age 73y, interquartile range 64-81y) were included. Pre-existing comorbidities were present in 90.1% of patients, and 21.9% had a history of > 1 malignancy. Half of the patients (n = 463, 49.3%) had received anticancer treatment ≤3 months before COVID-19 diagnosis (“active cancer”), of whom 286 (63.1%) in the non-curative setting. The overall 30-day and 3-month COVID-19-related mortality rates in this cohort were 21.4% (n = 178) and 24.1% (n = 194), respectively. COVID-related 3-month mortality was comparable in patients with active cancer (n = 96, 24.3%) and in patients with non-active cancer (n = 97, 24.0%), but within the first group COVID-related mortality was higher in those receiving systemic treatment in the non-curative (28.3%) versus the curative setting (15.2%). A change in the anticancer treatment plan due to COVID-19 was recorded in 148/463 patients with active cancer (32.0%). In patients with changes in systemic treatment plans (n = 146), treatment was delayed in 94 patients (in half of cases for > 1 month) and cancelled in 42 patients. The main reason for modifications in anti-cancer treatment was COVID-19 related complications (79.6%), followed by fear for/existence of anticancer treatment related toxicity (14.8%). Conclusions: Our nation-wide analysis in patients with solid cancer hospitalized with COVID-19 shows comparable 3-month mortality among patients who did and who did not receive anticancer treatment in the three months before COVID-19 diagnosis. Changes in anticancer treatment were very frequent in patients hospitalized with COVID-19. Further monitoring of the long-term impact of COVID-19-related changes to anticancer treatment plans is warranted.