A 40-year-old was seriously injured in an argument in Hamm early Saturday morning.
The man had been operated on during the night, said the police and prosecutors on Saturday.
In the meantime, the 40-year-old is out of danger, but not yet able to be questioned.
According to the public prosecutor's office, the act was apparently carried out with a "sharp weapon".
Hamm - Traces of blood were also found on the floor.
Further details were not known at first.
The police are investigating and looking for witnesses.
dpa