Although there is increased attention to designing and explaining clinical trials in ways that are clinically meaningful for patients, there is limited information on patient preferences, understanding, and perceptions of this content. Maximum difference scaling (MaxDiff) methodology was used to develop a survey for assessing patients' understanding of 19 clinical terms and perceived importance of 9 endpoint surrogate phrases used in clinical trials and consent forms. The survey was administered electronically to individuals with metastatic breast cancer affiliated with the Metastatic Breast Cancer Alliance. Analyses were performed using Bayesian P values with statistical software. Among 503 respondents, 77% had a college degree, 70% were diagnosed with metastatic disease ≥2 years before survey completion, and 77% had received ≥2 lines of systemic therapy. Less than 35% of respondents reported understanding "fairly well" the terms symptomatic progression, duration of disease control, time to treatment cessation, and endpoints. Income level and time since onset of metastatic disease correlated with comprehension. Patients who had received ≥6 lines of therapy perceived that time until serious side effects (P < .001) and time on therapy (P < .001) were more important compared with those who had received only 1 line of therapy. Positively phrased parameters were associated with increased perceived importance. Even among educated, heavily pretreated patients, many commonly used clinical research terms are poorly understood. Comprehension and the perceived importance of trial endpoints vary over the course of disease. These observations may inform the design, discussion, and reporting of clinical trials. © 2020 American Cancer Society.