SALEM — So long, Cuckoo’s Nest.

Oregon State Hospital, the mental institution where the 1975 movie “One Flew Over the Cuckoo’s Nest” was filmed, is making way for a new complex. Most of the dilapidated, 125-year-old main building will be torn down and replaced starting this fall.

Although mean Nurse Ratched was pure fiction, the Oregon State Hospital has struggled with some very real troubles over the years, including overcrowding, crumbling floors and ceilings, outbreaks of scabies and stomach flu, sexual abuse of children by staff, and patient-on-patient assaults.

Politicians had been talking for years about the need to replace the hospital, but didn’t get serious about it until a group of legislators made a grim discovery during a 2004 tour: the cremated remains of 3,600 mental patients in corroding copper canisters in a storage room. The lawmakers were stunned.

“Nobody said anything to anybody,” said Oregon Senate President Peter Courtney, who dubbed the chamber “the room of lost souls.”

The remains belonged to patients who died at the hospital from the late 1880s to the mid-1970s, when mental illness was considered so shameful that many patients were all but abandoned by their families in institutions.

“It just created such an emotional momentum” for replacing the hospital, said Courtney, who led the effort to build a new institution.

Although “Cuckoo’s Nest” was filmed here, neither the movie nor the 1962 Ken Kesey novel on which it was based makes any specific references to Oregon State Hospital. Kesey drew on his experiences working at a veterans hospital in Palo Alto, Calif., and set his satirical story at an unnamed institution in Oregon.

Actor Michael Douglas, co-producer of the movie, scouted various West Coast locations and chose the Oregon institution because then-Superintendent Dean Brooks agreed to give the moviemakers unfettered access.

“They wanted to make it on location with real patients,” said Brooks, now 91, who was given a speaking part as a weak-willed doctor who acquiesces to Nurse Ratched. Brooks said 89 patients were hired as extras.

Douglas, Jack Nicholson (who played the rebellious Randle Patrick McMurphy) and Louise Fletcher (Nurse Ratched) were regulars at the hospital during shooting.

Milos Forman, the director, lived for six weeks at the institution and had his actors study real patients, according to a 1975 account in Rolling Stone magazine. Nicholson became depressed because of what he saw, including electroshock being administered to a patient.

State leaders decided in 2006 to build a new, $300 million, 620-bed hospital at the site of the oldest and most dilapidated part of the complex, the J Building, a yellow-painted brick structure with brown trim, a towering cupola, and iron gratings on the windows.

The front section of the building, including the cupola, will be preserved as a museum on the history of mental health care.

Construction of the new hospital is set to begin next spring and should be completed by 2011.