Background: Little research has explored associations of longitudinal changes in PROs and clinical outcomes among EP-CT participants. We sought to describe changes in PROs over the first month of EP-CT participation and examine associations of changes with clinical outcomes. Methods: We prospectively enrolled adults participating in EP-CTs at Massachusetts General Hospital from 04/2021-01/2023. We assessed quality of life (QOL) (Functional Assessment of Cancer Therapy-General, with subdomains: physical wellbeing [PWB], emotional wellbeing [EWB], social wellbeing [SWB], functional wellbeing [FWB]); hope (Herth Hope Index); symptom burden (Edmonton Symptom Assessment System); and financial wellbeing (Comprehensive Score for Financial Toxicity) at time of EP-CT enrollment and one month later (1-month). We used descriptive statistics and regression models to describe 1-month changes in PROs and explore associations of patient-reported QOL, hope, symptom burden, and financial wellbeing with patient factors (age, diagnosis, performance status) and clinical outcomes (time on trial [TOT], treatment response, ED visits, hospitalizations, and survival). Results: Of 205 enrolled patients, 159 (77.9%) had baseline and 1-month data evaluable for analysis (median age: 63.0 [range 33.0 – 88.6], 59.8% female, 93.6% metastatic cancer, ECOG 0 = 46.2% and ECOG 1 = 53.8%). Most common cancer types were gastrointestinal (31.5%) and breast (22.0%). There was no significant change from baseline to 1-month in mean QOL (75.2 to 76.2), hope (27.1 to 26.7), ESAS total symptom burden (20.4 to 20.9), ESAS physical symptoms (13.3 to 13.7), and financial wellbeing (28.5 to 28.0). There was no significant change in overall QOL and in each subdomain with the exception of EWB (16.2 to 17.0, p < .001). Older age was associated with increased hope (B = 1.29, p = .008) and increased QOL (B = 3.02, p = 0.024). Increasing EWB at 1-month was associated with longer TOT (HR = 0.93, p = .011). Increased symptom burden at 1-month was associated with greater risk of ED visits (HR = 1.04, p = .005) and hospitalizations (HR = 1.03, p = .012). Improving financial wellbeing at 1-month was associated with lower risk of hospitalization (HR = 0.96, p = .046). We found no other significant associations of 1-month changes in PROs with treatment response or survival. Conclusions: We described changes in PROs from baseline to 1-month and identified factors associated with these changes. Despite the rigors and intensity associated with EP-CT initiation, most PROs did not worsen, with significantly improved EWB after 1-month of participation. Results demonstrate changes in physical symptoms predicted for greater risk of ED visits and hospitalizations. Future studies should build upon this work and measure PROs throughout trial participation to identify factors associated with changes and guide intervention development.