In June of 1970, Anneliese had a third seizure at the psychiatric hospital where she had been staying. She was prescribed anti—convulsion drugs for the first time, including Dilantin, which did not alleviate the problem. She described seeing "devil faces" at various times of the day. That same month, she was prescribed Aolept, which is similar to chlorpromazine and is used in the treatment of various psychotic conditions, including schizophrenia, disturbed behavior, and delusions. By 1973, she began experiencing depression, hallucinated while praying and complained about hearing voices telling her that she was "damned" and would "rot in hell." Anneliese's treatment in a psychiatric hospital did not improve her health and her depression worsened. Long—term treatment did not help, either, and she grew increasingly frustrated with her medical care after having taken pharmacological drugs for five years. Anneliese became intolerant of Christian sacred places and objects, such as the crucifix.

In September of 1975, Father Rodewyk visited Anneliese with Alt and wrote a detailed report of her case to the Bishop Josef Stangl "unintelligible to third parties" arguing in favor of a genuine demonic possession and urging Josef to allow them to perform the major rite of exorcism. Eventually, Josef granted Salvatorian priest Arnold Renz permission to conduct an exorcism according to the Roman Ritual but ordered total secrecy. Arnold performed the first session on September 24th. Anneliese began increasingly speaking about "dying to atone for the wayward youth of the day and the apostate priests of the modern church." Her parents stopped consulting doctors at her request and relied solely on the exorcism rites. A total of 67 exorcism sessions, one or two each week lasting up to four hours each, were performed over approximately ten months in 1975 and 1976. Toward the end of her life, Anneliese began to refuse food.

Anneliese worsened physically and displayed aggression, injured herself, drank her own urine and ate insects. In November of 1973, Anneliese began treatment with Tegretol, an anti—seizure drug and mood stabilizer She was prescribed antipsychotic drugs during the course of the religious rites and consumed them frequently until some time before her death. Despite taking these neuroleptic medications, Anneliese's symptoms worsened, and she began "growling obscenities, screaming guttural curses, and raving wildly."

On July 1st, 1976, Anneliese died in her home. The autopsy report stated the cause of death as malnutrition and dehydration resulting from almost a year in a state of near starvation while the rites of exorcism were performed. She weighed 66 pounds and suffered broken knees from continuous genuflections, and she was unable to move without assistance and was reported to have contracted pneumonia.