Patient-reported health data can facilitate clinical event capture in pragmatic clinical trials. However, few data are available on the fitness for use of patient-reported data in large-scale health research. To evaluate the concordance of a set of variables reported by patients and available in the electronic health record as part of a pragmatic clinical trial. Data from ADAPTABLE (Aspirin Dosing: A Patient-Centric Trial Assessing Benefits and Long-term Effectiveness), a pragmatic clinical trial, were used in a concordance substudy of a comparative effectiveness research trial. The trial randomized 15 076 patients with existing atherosclerotic cardiovascular disease in a 1:1 ratio to low- or high-dose aspirin from April 2016 through June 30, 2019. Concordance of data was evaluated from 4 domains (demographic characteristics, encounters, diagnoses, and procedures) present in 2 data sources: patient-reported data captured through an online portal and data from electronic sources (electronic health record data). Overall agreement, sensitivity, specificity, positive predictive value, negative predictive value, and κ statistics with 95% CIs were calculated using patient report as the criterion standard for demographic characteristics and the electronic health record as the criterion standard for clinical outcomes. Of 15 076 patients with complete information, the median age was 67.6 years (range, 21-99 years), and 68.7% were male. With the use of patient-reported data as the criterion standard, agreement (κ) was high for Black and White race and ethnicity but only moderate for current smoking status. Electronic health record data were highly specific (99.6%) but less sensitive (82.5%) for Hispanic ethnicity. Compared with electronic health record data, patient report of clinical end points had low sensitivity for myocardial infarction (33.0%), stroke (34.2%), and major bleeding (36.6%). Positive predictive value was similarly low for myocardial infarction (40.7%), stroke (38.8%), and major bleeding (21.9%). Coronary revascularization was the most concordant event by data source, with only moderate agreement (κ = 0.54) and positive predictive value. Agreement metrics varied by site for all demographic characteristics and several clinical events. In a concordance substudy of a large, pragmatic comparative effectiveness research trial, sensitivity and chance-corrected agreement of patient-reported data captured through an online portal for cardiovascular events were low to moderate. Findings suggest that additional work is needed to optimize integration of patient-reported health data into pragmatic research studies. ClinicalTrials.gov Identifier: NCT02697916.