| 1894 | Dr. Harry W. Still (son of A.T. Still, founder of osteopathic medicine) was the first DO to establish a permanent practice in Illinois. |
| 1899 | The first legislative recognition of Osteopathy in Illinois. DOs were examined by the Medical Board as “Other Practitioners”, could not be called doctors, and had no legal rights to be a physician. |
| 1900 | American College of Osteopathic Medicine and Surgery was founded in Chicago. |
| 1902 | Chicago School of Osteopathy was founded in Chicago. |
| 1903 | The Illinois Osteopathic Association was incorporated in Illinois. |
| 1904 | The Chicago School of Osteopathy was absorbed by the American College of Osteopathic Medicine and Surgery. |
| 1913 | The American College of Osteopathic Medicine and Surgery changed its name to the Chicago College of Osteopathy. |
| 1913 | A.T. Still Research Institute laboratories opens in Chicago. It closes in 1918. |
| 1920 | First meeting of the AOA House of Delegates held in June at the Sherman Hotel in Chicago. |
| 1922 | AOA headquarters moved to Chicago. |
| 1923 | Illinois law was changed to allow Doctors of Osteopathic Medicine to be examined in—and legally practice—Obstetrics. |
| 1952 | The Articles of Incorporation of the Illinois Association of Osteopathic Physicians and Surgeons was filed with the Illinois Secretary of State. The IAOPS would become the Illinois Osteopathic Medical Society (IOMS) in 1999. |
| 1955 | The Illinois Supreme Court hands down a decision approving the Chicago College of Osteopathy as a school qualified to teach medicine and surgery in all of its branches. |
| 1999 | The Illinois Association of Osteopathic Physicians and Surgeons changed its name to the Illinois Osteopathic Medical Society. |
