Most trainees in combined pulmonary and critical care medicine fellowship programs complete in-service training examinations (ITEs) that test knowledge in both disciplines. Whether ITE scores predict performance on the American Board of Internal Medicine Pulmonary Disease Certification Examination and Critical Care Medicine Certification Examination is unknown. To determine whether pulmonary and critical care medicine ITE scores predict performance on subspecialty board certification examinations independently of trainee demographics, program director competency ratings, fellowship program characteristics, and prior medical knowledge assessments. First- and second-year fellows who were enrolled in the study between 2008 and 2012 completed a questionnaire encompassing demographics and fellowship training characteristics. These data and ITE scores were matched to fellows' subsequent scores on subspecialty certification examinations, program director ratings, and previous scores on their American Board of Internal Medicine Internal Medicine Certification Examination. Multiple linear regression and logistic regression were used to identify independent predictors of subspecialty certification examination scores and likelihood of passing the examinations, respectively. Of eligible fellows, 82.4% enrolled in the study. The ITE score for second-year fellows was matched to their certification examination scores, which yielded 1,484 physicians for pulmonary disease and 1,331 for critical care medicine. Second-year fellows' ITE scores (β = 0.24, P < 0.001) and Internal Medicine Certification Examination scores (β = 0.49, P < 0.001) were the strongest predictors of Pulmonary Disease Certification Examination scores, and were the only significant predictors of passing the examination (ITE odds ratio, 1.12 [95% confidence interval, 1.07-1.16]; Internal Medicine Certification Examination odds ratio, 1.01 [95% confidence interval, 1.01-1.02]). Similar results were obtained for predicting Critical Care Medicine Certification Examination scores and for passing the examination. The predictive value of ITE scores among first-year fellows on the subspecialty certification examinations was comparable to second-year fellows' ITE scores. The Pulmonary and Critical Care Medicine ITE score is an independent, and stronger, predictor of subspecialty certification examination performance than fellow demographics, program director competency ratings, and fellowship characteristics. These findings support the use of the ITE to identify the learning needs of fellows as they work toward subspecialty board certification.