© 2017 by the American Association for the Study of Liver Diseases. This article has been contributed to by US Government employees and their work is in the public domain in the USA. The Centers for Disease Control and Prevention and US Preventive Services Task Force recommend one-time hepatitis C virus (HCV) testing for persons born during 1945-1965 (birth cohort). However, few studies estimate the effect of birth cohort (BC) testing implementation on HCV diagnoses in primary care settings. We aimed to determine the probability of identifying HCV infections in primary care using targeted BC testing compared with usual care at three academic medical centers. From December 2012 to March 2014, each center compared one of three distinct interventions with usual care using an independently designed randomized controlled trial. Across centers, BC patients with no clinical documentation of previous HCV testing or diagnosis were randomly assigned to receive a one-time offering of HCV antibody (anti-HCV) testing via one of three independent implementation strategies (repeated-mailing outreach, electronic medical record–integrated provider best practice alert [BPA], and direct patient solicitation) or assigned to receive usual care. We estimated model-adjusted risk ratios (aRR) of anti–HCV-positive (anti-HCV+) identification using BC testing versus usual care. In the repeated mailing trial, 8992 patients (intervention, n = 2993; control, n = 5999) were included in the analysis. The intervention was eight times as likely to identify anti-HCV+ patients compared with controls (aRR, 8.0; 95% confidence interval [CI], 2.8-23.0; adjusted probabilities: intervention, 0.27%; control, 0.03%). In the BPA trial, data from 14,475 patients (BC, n = 8928; control, n = 5,547) were analyzed. The intervention was 2.6 times as likely to identify anti-HCV+ patients versus controls (aRR, 2.6; 95% CI, 1.1-6.4; adjusted probabilities: intervention, 0.29%; control, 0.11%). In the patient-solicitation trial, 8873 patients (BC, n = 4307; control, n = 4566) were analyzed. The intervention was five times as likely to identify anti-HCV+ patients compared with controls (aRR, 5.3; 95% CI, 2.3-12.3; adjusted probabilities: intervention, 0.68%; control, 0.11%). Conclusion: BC testing was effective in identifying previously undiagnosed HCV infections in primary care settings. (Hepatology 2018;67:524-533).