Background: Use of digital tools in cancer care, including those for collecting patient reported outcomes (PRO), is increasing. While ePRO has been shown to reduce site burden and offer flexibility to patients, examining ePRO uptake is important to inform future implementation and identify disparities. We evaluated factors associated with ePRO use in a prospective study examining predictors of IR-AEs in patients with solid tumors treated with immune checkpoint inhibitors. Methods: We used data from the SWOG Cancer Research Network trial S2013 (NCT04871542). At enrollment, patients completed baseline PROs on paper and were offered the option to complete follow-up PROs at prespecified times on paper or using a mobile ePRO application. Patients enrolled from study activation in August 2021 to June 12, 2023 were analyzed. We examined ePRO uptake overall and by age (<65 v. ≥65 years), sex (male v. female), race (White v. Non-White), Hispanic ethnicity (yes v. no), and performance status (PS; 0 v. ≥1). For patients <65 years, we examined ePRO use by insurance [private/other v. Medicaid/no insurance (M/NI)]. Factors that were statistically significant associated with ePRO use were used to calculate a risk score for non-use of ePRO, with levels of each factor coded as 0 (absence of risk) or 1 (presence). Total risk score was a sum of risk factors. Patients were classified based on the median risk score, and the association of risk level with ePRO use was examined. Two-sided Chi-squared tests at alpha=.05 were used for all comparisons. Results: We examined 1,118 patients with a median age of 69.5 years. 37.6% of patients were female and 90.2% were White. Only 9.3% of patients (N=104) opted to use ePRO. Patients <65 years were more likely to use ePRO (13.9% v. 6.9%; p=.0001) as were patients with PS=0 (13.2% v. 6.5%; p=.0002). ePRO use was higher among patients of White race (9.5% v. 3.8%; p=.09) and non-Hispanic ethnicity (9.8% v. 3.4%; p=.10) but the differences were not statistically significant. For patients <65 years, ePRO use was higher for those with private/other insurance compared to those with M/NI (17.0% v. 5.3%; p=.01). The risk score (range: 0-3) for all patients was based on age, PS, and insurance (private/other/Medicare v. M/NI), with 52% of patients classified as low risk (score = 0 or 1). Patients with a low risk score had higher ePRO uptake than high-risk patients (13.1% v. 5.7%; p<.0001). Conclusions: The option to complete PROs on paper remains important for many patients. Few patients currently choose to use ePRO with uptake rates higher for patients who are younger, have private insurance, and better physical function. Presence of multiple adverse risk factors reduces the likelihood of using ePRO. Access to and familiarity with technology and presence of physical disabilities may be some factors driving these disparities. Future work will examine area-level factors associated with ePRO use.