Background: Editorials accompanying the publication of trials in major oncology journals can have a substantial influence on clinical practice. We describe the prevalence of financial conflicts of interest (FCOIs) of authors writing such editorials and the extent to which FCOIs may shape the interpretation of clinical trials. Methods: We examined editorials published in 2018 alongside trial reports in the top 5 journals that publish cancer drug trials (New England Journal of Medicine, Lancet, Lancet Oncology, JAMA Oncology, and Journal of Clinical Oncology). An editorial was considered to have an FCOI if at least one of the editorialists had any disclosed FCOI. An FCOI with the same company whose drug was being discussed in the editorial was classified as a direct FCOI. Editorials were reviewed for their content and classified as being unduly favorable (defined as the presence of a positive spin without discussion of limitations) or not. Association of an FCOI and a direct FCOI with writing an unduly favorable editorial was assessed. Results: Of the 90 editorials assessed, 74% (n=67) were classified as having an FCOI with the pharmaceutical industry, and 39% (n=35) had an FCOI with the same company whose product was being discussed in the editorial (direct FCOI). Editorials were classified as being unduly favorable toward the study drug in 12% (8 of 67) and 13% (3 of 23) (P=1.0) of those with and without FCOIs, respectively; corresponding rates with and without direct FCOI were 23% (8 of 35) and 5% (3 of 55), respectively (P=.009). Conclusions: Editorials in top oncology journals were frequently authored by experts with FCOIs, including direct FCOIs. Authoring an unduly favorable editorial for a new cancer drug was significantly associated with the author having a direct FCOI with the same company. These findings support the call for journals to ensure that authors of editorials have no direct FCOIs.