Background <p>Early phase clinical trials (EP-CTs) investigate novel therapeutic approaches for patients with cancer, but little is known about patterns of supportive care service utilization and advance care planning (ACP) in this population. We sought to characterize these features in an EP-CT population and evaluate associations among receipt of supportive care services and ACP documentation.</p> Methods <p>We retrospectively reviewed the electronic health record (EHR) of consecutive patients enrolled in EP-CTs at Massachusetts General Hospital from 01/01/17–12/30/19. We abstracted sociodemographics, performance status (Eastern Cooperative Oncology Group [ECOG] score), oncology history, trial details, as well as receipt and timing of six supportive care services (palliative care [PC], social work [SW], spiritual services [SS], parental support [PS], physical therapy [PT], and nutrition). We additionally abstracted receipt and timing of ACP documentation (defined as any EHR-documented conversation addressing illness understanding or values, preferences, or goals for future medical care, as identified using a structured keyword search). We then separately examined associations between receipt of any supportive care service and ACP documentation, number of supportive care services received and ACP documentation, and subtype of supportive care received and ACP documentation. These analyses used logistic regression models adjusted for age, sex, cancer type, and performance status.</p> Results <p>During our study period, 376 patients participated in EP-CTs (median age 63.0&#xa0;years, 55.9% female, 97.3% stage 4, median ECOG 1, median follow-up: 223&#xa0;days, median time from diagnosis to EP-CT: 844&#xa0;days). Nearly all received at least one type of supportive care across their illness trajectory (88.0%), with varied rates by service type (PC: 54.8%, SW: 64.1%, SS: 39.1%, PS: 8.0%, PT: 54.0%, nutrition: 61.2%). Most also had some form of ACP (73.9%) documented between diagnosis and death. Multivariable regression models demonstrated that receipt of any of the six forms of supportive care was associated with higher likelihood of ACP documentation (odds ratio [OR]: 9.18, 95% confidence interval (CI): 4.49–18.78, p &lt; 0.001). Similarly, we observed associations between number of supportive care services received when considered as a continuous covariate and ACP documentation (OR_{1 service}:1.89, 95%CI:0.90–4.03, <i>p</i> = 0.090; OR_{2 services}: 15.36, 95%CI 5.78–40.78, p &lt; 0.001, OR_{3+ services}: 35.78, 95%CI: 14.35–89.24, p &lt; 0.001). These associations also persisted when considering PC independently (OR_PC = 11.17, 95%CI_PC = 5.76–21.67, p &lt; 0.001) from other supportive care services (OR_other = 5.41, 95%CI_other: 2.64–11.09, p &lt; 0.001).</p> Conclusions <p>In this large cohort of EP-CT participants, most patients received supportive care services and had documented ACP, suggesting trial-related engagement does not impede care delivery. Notably, receipt of supportive care services correlated with ACP documentation. These findings underscore the importance of addressing individual supportive care needs among EP-CT participants.</p>