The American Cancer Society (ACS) and Coalition of Cancer Cooperative Groups (CCCG) provide a clinical trial (CT) information/matching/eligibility service (Clinical Trials Matching Service [CTMS]). Patients' demographic and clinical data, enrollment status, and self-reported barriers to CT participation were analyzed to assess enrollment rates and determinants of enrollment. During 3 years beginning October 1, 2007, the CTMS served 6,903 patients via the ACS call center. Among the 1,987 patients with follow-up information on enrollment, 219 (11.0%) enrolled in a CT; 48 of these 219 enrollees chose a CT they found via the CTMS. Patients were less likely to enroll if they had poor ECOG performance status (P = 0.032); were African American (P = 0.0003), were uninsured or had Medicaid coverage (P = 0.024), or had lower stage disease (P = 0.018). Enrollment varied by trial type/cancer site/system (P = .026). Several barriers significantly predicted nonenrollment. Broader availability of a CTMS might help improve patient participation in cancer clinical trials.