The Accreditation Council of Graduate Medical Education (ACGME) developed the Milestones to assist training programs in assessing resident physicians in the context of their participation in ACGME-accredited training programs. Bi-annual assessments are done over a resident's entire training period to define the trajectory in achieving specialty-specific competencies. As part of its process of initial certification, the American Board of PM&R (ABPMR) requires successful completion of two examinations administered about nine months apart. The Part I Examination measures a single dimensional construct, PM&R medical knowledge (MK), while Part II assesses the application of medical and physiatric knowledge to multiple domains, including data acquisition, problem solving, patient management, systems-based practice and interpersonal and communication skills through specific patient case scenarios. This study aims to investigate the validity of the Milestones by demonstrating its association with performance in the ABPMR certifying examinations. A cohort of 233 PM&R trainees in 3-year residency programs (PGY-2 entry) in the United States from academic years (AY) 2014-2016, who also took the ABPMR Parts I and II certifying examinations between 2016 and 2018, were included in the study. Milestones ratings in four distinct observation periods were correlated with scores in the ABPMR Parts I and II Examinations. Milestones ratings of MK (but not patient care, professionalism, problem-based learning, interpersonal and communication skills and systems-based practice) predicted performance in subsequent Part I ABPMR Examination, but none of the Milestone ratings correlated with Part II Examination scaled scores.