American College of Physicians | 39,228 followers on LinkedIn. Leading Internal Medicine, Improving Lives | The American College of Physicians is the largest medical specialty organization in the United States with members in more than 172 countries worldwide. ACP membership includes 163,000 internal medicine physicians, related subspecialists, and medical students. Internal medicine physicians are specialists who apply scientific knowledge and clinical expertise to the diagnosis, treatment, and compassionate care of adults across the spectrum from health to complex illness.