The daughter, 55, was admitted to a psychiatric hospital after police made the grisly find in the southern city of Munich, the Sueddeutsche Zeitung daily reported.
A social worker had attempted to visit the mother last Thursday after authorities had been rebuffed several times but was denied entry by the daughter.
The worker then alerted police who asked the fire brigade to open the door to the flat.
Police said other tenants in the apartment block had in the past reported a “strange smell” and had told building management that they had not seen the older woman for some time.
Officers who questioned the daughter and ordered a post-mortem examination concluded that the mother died of natural causes in March 2009 at the age of 77.
They were shocked to find that the daughter had shared a bed with her mother’s corpse. -AFP