The woman was arrested on Monday on suspicion of "unlawful deprivation of liberty" and "causing grievous bodily harm".
ADVERTISEMENTA prosecutor in Sweden says there is no evidence to support suspicions a 70-year-old mother imprisoned her adult son for nearly three decades.
On Monday, the woman was arrested on suspicion of "unlawful deprivation of liberty" and "causing grievous bodily harm" after a relative found the man in a flat in the southern suburbs of the Swedish capital Stockholm.
Her son was found with injuries, such as bruises and sores, in a flat which was described as full of rubbish and filth.
But on Thursday, Sweden's chief prosecutor said the case would be closed.
"We found no evidence that he was locked up, tied up, or physically prevented from leaving the premises," said Emma Olsson.
"The man himself said that it was up to him to decide whether he wanted to leave the flat," she added, adding that witnesses had occasionally seen him outside.
Prosecutors have also said that the 41-year-old man's injuries could not be attributed to inflicted violence.
Emma Olsson said in a statement that "suspicions against the suspect woman have weakened" after her son had been questioned by the authorities.
Prosecutors said her son's injuries were due to his ill-health and not violence. He remains in hospital.
Social services in Sweden are now investigating the case, following reports that the son was taken out of school at a very young age and then isolated.
"It is questionable whether he is fit to be a mother, but it is now up to the social services to look into this issue," the prosecutor said.
"Society must now help this man, and this woman too. And make sure that we can prevent this kind of thing from happening again," she added.