Background: Patients participating in phase I trials represent a population with advanced cancer and symptoms, with quality-of-life implications arising from both disease and treatment. Transitions to end-of-life care for these patients have received little attention. Good empirical data are needed to better understand the role of advance care planning and palliative care during phase I trial transitions. We investigated how physician–patient communication at the time of disease progression, patient characteristics, and patterns of care were associated with end-of-life care. Methods: We conducted a retrospective chart review of all patients with solid tumors enrolled in phase I trials at a comprehensive cancer center from January 2015 to December 2017. We captured physician–patient communication during disease progression. Among patients who died, we assessed palliative care referral, advance care planning, place of death, healthcare use in the final month of life, hospice enrollment, and hospice length of stay (LOS). Factors independently associated with a short hospice LOS (defined as ≤3 days) were estimated from a multivariable model building approach. Results: Among 207 participants enrolled in phase I intervention studies at Johns Hopkins Hospital, the median age was 61 years (range, 31–91 years), 48% were women, 21% were members of racial minority groups, and 41.5% were referred from an outside institution. At the time of disease progression, 53% had goals of care documented, 47% were previously referred to palliative care, and 41% discussed hospice with their oncologist. A total of 82% of decedents died within 1 year of study enrollment, and 85% enrolled in hospice. Among the 147 participants who enrolled in hospice, 22 (15%) had a short LOS (≤3 days). Factors independently associated with an increased risk of short hospice LOS in the multivariable model included age >65 years (odds ratio [OR], 1.12; 95% CI, 1.01–1.24; P=.04), whereas remaining at the same institution (OR, 0.72; 95% CI, 0.65–0.80; P<.001) and referral to palliative care before progression (OR, 0.83; 95% CI, 0.75–0.92; P<.001) were associated with a decreased risk of short hospice LOS. Conclusions: Reported data support the benefit of palliative care for patients in phase I trials and the risks associated with healthcare transitions for all patients, particularly older adults, regardless of care received. Leaving a clinical trial is a time when clear communication is paramount. Phase I studies will continue to be vital in advancing cancer treatment. It is equally important to advance the support provided to patients who transition off these trials.