Originally one of two mental hospitals planned for the area, this particular one was the oldest and largest. The buildings were laid out in a cross-like pattern, with services such as administration, workshops, kitchen and bath house placed along the axis, and wards perpendicular to these. Gardens were symmetrically planted between the structures, and the treatment areas were separated by gender.

In May 1939, after Hitler spoke of the mentally ill who were "life unworthy of life," physicians began killing children who had physical and mental disabilities. Only two months later, plans were already underway to kill institutionalized adults, all under the Aktion T4 program. In October, all nursing homes, hospitals, and sanatoria were required to report every patient who met a very broad set of ailments, including schizophrenia, epilepsy, advanced syphilis, senile dementia, paralysis, and encephalitis. Many doctors thought these reports were being used to determine who was healthy enough to work under the labor service, and exaggerated the conditions of many in hopes to keep them in the hospital, but resulting in fatal consequences. These patient reports were reviewed by a board of "experts," who used the paperwork alone to determine if the person should live or be exterminated; as time went on, the thoroughness of these paper examinations decreased, and the range of required conditions became even broader.

Once a patient had received three minus symbols from the three people on the panel, they were administered a lethal injection (the same process for killing the children), however the cost of the drugs were quite expensive. Once the carbon monoxide gas chamber became the preferred method upon Hitler's recommendation, the patients were moved from the hospitals to "special treatment" centers by the busload, where they were gassed and cremated. Coherent patients were told they were going to a special clinic where improved treatment would be given. They were given a brief examination, then led to what appeared to be a large shower room where the gas was administered - a method that would be used later on in the concentration camps. Families were given a letter describing that the patient was being transferred to a special treatment center, and that they would not be able to visit due to wartime circumstances; the next letter was a death certificate with a phony but plausible cause, and random ashes from the en-masse cremation.

Action T4 was canceled under Hitler's order in August 1941 due to overwhelming pressure from the Catholic church, as well as from protests around the country after hearing of the program through illegally printed material. The cancellation of the program ended the systematic gas killings, however institutionalized adults and children were still being given lethal injections and subjected to starvation until the end of the war. The Nuremberg Trials had determined that somewhere around 275,000 people were killed under T4.

After the war, the hospital was almost completely empty after having lost so much of the patient population due to the Action T4 extermination, but sections of the campus were eventually rehabilitated back into psychiatric services, general hospital care, and other uses. Although some are empty at the moment, a new hospital is planning to utilize these abandoned spaces.

* Note: the name "Irrenanstalt Weiler" is a pseudonym; the real name of this location is currently undisclosed.