BACKGROUND: Clinical trial protocols often allow investigators discretion to determine whether individuals are eligible to participate. OBJECTIVE: To examine the frequency and impact of discretionary eligibility criteria in oncology clinical trials, including whether they specifically target populations with psychiatric or cognitive disabilities for exclusion. METHODS: We identified interventional Phase III and Phase IV oncology clinical trials on ClinicalTrials.gov starting between January 1, 2019, and December 31, 2023. Among these, we included trials with at least one US location, patient populations aged 18-65 or 65+, and a posted study protocol. We used descriptive statistics to capture prevalence of broad investigator discretion in eligibility criteria. We reviewed statements concerning discretionary eligibility criteria for text relating to psychiatric or cognitive disability, and we applied qualitative content analysis techniques to identify themes. RESULTS: Ninety-six trials met inclusion criteria for this study. Of these, 82 (85.4 %) of trials allowed broad investigator discretion to determine eligibility. Content analysis revealed concerns about participants' safety (i.e., posing undue risk to patients), potential difficulties adhering to study protocols (e.g., because of patients' physical, psychiatric, or social condition), perceived ability to obtain informed consent, and completing assessments of patients to determine results of interventions. All trials required informed consent. CONCLUSIONS: Most oncology clinical trials allow investigators broad discretion in determining the eligibility of individuals to participate. These discretionary criteria may particularly target persons with psychiatric or cognitive disabilities, excluding them from participating in clinical trials. Further research should examine whether certain rationales for excluding these populations are appropriate.