Between 1985 and 2006, the number of osteopathic physicians (DOs) training in Accreditation Council for Graduate Medical Education (ACGME) postdoctoral (i.e., residency and fellowship) programs increased by 5,352 (419%). In 2006, more than two of every three DOs (6,629 of 9,618) in postdoctoral training were in an ACGME program. The integration of osteopathic physicians into these programs was facilitated by several factors. The most important of these was a noted growth in the number and types of ACGME programs and a consistent number of U.S. MD graduates (USMDs) from schools accredited by the Liaison Committee on Medical Education (LCME). From 1985 to 2006, the number of all physicians in ACGME programs, both DO and MD, grew by 30,365 (41%). DOs were most often selected for specialties less populated by USMDs, chiefly family and internal medicine and pediatrics.Growth patterns in LCME medical schools project an increase in the national class size to accommodate 3,400 more students by 2012, a 21% increase. The development of new colleges of osteopathic medicine (COMs) and expansion in existing ones is expected to generate 5,227 first-year students in 2012, an increase of 1,380 students (36%) over 2006 numbers. The overwhelming majority of these anticipated new COM graduates cannot be accommodated in American Osteopathic Association postdoctoral programs because of limited capacity. As these additional LCME graduates move into their postdoctoral training, educational opportunities for DOs are expected to decline and competition is expected to become stiffer. The window of opportunity for DOs in ACGME programs that opened in the last two decades will gradually start to close.