The Accreditation Council for Graduate Medical Education's Next Accreditation System requires training programs to demonstrate that fellows are achieving competence in medical knowledge (MK), as part of a global assessment of clinical competency. Passing American Board of Internal Medicine (ABIM) certification examinations is recognized as a metric of MK competency. This study examines several in-training MK assessment approaches and their ability to predict performance on the ABIM Hematology or Medical Oncology Certification Examinations. Results of a Hematology In-Service Examination (ISE) and an Oncology In-Training Examination (ITE), program director (PD) ratings, demographic variables, United States Medical Licensing Examination (USMLE), and ABIM Internal Medicine (IM) Certification Examination were compared. Stepwise multiple regression and logistic regression analyses evaluated these assessment approaches as predictors of performance on the Hematology or Medical Oncology Certification Examinations. Hematology ISE scores were the strongest predictor of Hematology Certification Examination scores (β = 0.41) (passing odds ratio [OR], 1.012; 95 % confidence interval [CI], 1.008-1.015), and the Oncology ITE scores were the strongest predictor of Medical Oncology Certification Examination scores (β = 0.45) (passing OR, 1.013; 95 % CI, 1.011-1.016). PD rating of MK was the weakest predictor of Medical Oncology Certification Examination scores (β = 0.07) and was not significantly predictive of Hematology Certification Examination scores. Hematology and Oncology ITEs are better predictors of certification examination performance than PD ratings of MK, reinforcing the effectiveness of ITEs for competency-based assessment of MK.