The American Thoracic Society (ATS), the preeminent professional organization in the field of respiratory, critical care, and sleep medicine, is now 107 years old. For the most part, the Society's administrative and medical-scientific interests evolved in an orderly fashion, but two "revolutions" took place that should be remembered. What ultimately metamorphosed into the ATS in 1960 began in 1905 as the 34-member American Sanatorium Association, which in 1915 became the medical section of the National Association for the Study and Prevention of Tuberculosis (NASPT). In 1918, the NASPT became the National Tuberculosis Association and in 1939, the ASA became the American Trudeau Society, cosmetic revisions having no effect on either the medical section-parent relationship or the one-disease orientation of both organizations. After World War II, the narrow focus of the ATS on tuberculosis was progressively enlarged through coalescence of several factors that transformed the practice of pulmonary medicine: the growth of intensive care units and pulmonary function laboratories and the advent of fiberoptic bronchoscopy; the rise of asthma, chronic obstructive pulmonary disease, and lung cancer coincident with the withering of tuberculosis; and the arrival of pulmonary physician-scientists who sought enrichment through a professional society. The newcomers found a home in the ATS, but it was slow to fulfill their needs for scientific communication and administrative responsibility. The first revolution, the formation of Scientific Assemblies, got the job done quickly and well, as described in Part 1 of this perspective. The second revolution, separation from the American Lung Association, is described in Part 2.