Background: Disparities in clinical trial participation beyond first-line (1L) treatment of metastatic cancer have not been well studied. Using real-world data from patients diagnosed with metastatic breast cancer (mBC) or metastatic colorectal cancer (mCRC), this study evaluated 1) patterns of participation in trials by treatment line and 2) factors associated with participation in trials of second-line (2L) and subsequent lines of treatment. Methods: Patients diagnosed with mBC or mCRC between 2013 and 2021 who received at least 2L treatment were retrospectively identified from the Flatiron Health electronic health record database. Clinical trial participation was defined as receipt of “clinical study drug.” Participation in trials for treatment lines 2L to 6L were assessed, and multivariable logistic regression models were used to evaluate associations between sociodemographic and clinical factors, including age, race/ethnicity, socioeconomic status (SES), stage at diagnosis, year of metastatic diagnosis, practice type, and total lines of treatment, with participation in trials of 2L and subsequent lines of treatment. Results: Overall, participation in trials was low for patients with mBC or mCRC. For patients with mBC (n=14,590), participation in trials was similar from 2L (2.6%) to 6L (2.7%) (Table 1). Among these patients, age, race/ethnicity, and practice type were associated with participation in trials, while SES was not. For example, women age 18-49, 50-64, and 65-74 years were more likely to participate than women 75 or older (aORs: 2.88, 2.45, 1.84), and non-Hispanic Asian, non-Hispanic Black, and Hispanic women were less likely to participate than non-Hispanic white women (aORs: 0.41, 0.47, 0.61). In contrast to patients with mBC, participation in trials increased from 2.0% (2L) to 6.6% (6L) for patients with mCRC (n=7,303; Table 1). Among these patients, age, practice type, and SES were associated with participation in trials. Specifically, patients treated in academic compared to community settings (aOR: 4.25, 95% CI: 3.33, 5.42) and patients with higher SES (aOR: 2.00, 95% CI: 1.29, 3.08) were more likely to participate. For both patients with mBC (aOR: 5.00, 95% CI, 3.87, 6.46) or mCRC (aOR: 7.91, 95% CI: 5.63, 11.10), those who received more than five treatment lines compared to only two treatment lines were more likely to participate in a trial. Conclusions: These real-world findings highlight disparities in participation in clinical trials of 2L and subsequent lines of treatment by age, race/ethnicity, practice type, and SES among patients with mBC or mCRC. Further policy efforts and interventions are needed to address systematic barriers to increase participation of underrepresented groups in clinical trials.