This study examines patients' understanding of health insurance terms and concepts and quantifies health insurance literacy (HIL) levels by key sociodemographic factors. This study included 393 adult patients with cancer (>18 years old) receiving treatment in two ambulatory infusion centers: Mayo Clinic in Phoenix, Arizona and the University of Mississippi Medical Center in Jackson, Mississippi. Respondents' perceptions of their HIL were assessed using the Health Insurance Literacy Measure (HILM), a validated 21-item measure of a consumer's ability to select and use health insurance (HIL self-efficacy). Respondents' knowledge of health insurance concepts (HIL knowledge) was measured using 10 items created by the Kaiser Family Foundation. The number of correct answers was categorized into three levels: 0-4 (low knowledge), 5-6 (moderate knowledge), and 7-10 (high knowledge). Multivariable logistic regressions were used to compare correct answers to HIL knowledge questions by HIL self-efficacy. Nearly three-quarters of patients had high HIL self-efficacy and high HIL knowledge (70.5%), understanding basic insurance terms, such as premiums and deductibles. Relatively low percentages of patients correctly answered questions about the meaning of provider networks, health insurance formularies, and calculating out-of-pocket spending in scenarios when insurers pay a portion of allowed charges. Lower HIL knowledge was more common among patients with less educational attainment (<college: 15.2%; ≥college: 4.7%), lower household income (<$4999: 11.8%; ≥$5000: 6.1%), and low HIL self-efficacy scores (low: 24.8%; high: 5.2) (all p values <.05). Efforts to improve HIL and navigation of health insurance plan features are required, especially for socioeconomically vulnerable patients.