A German passenger stripped naked on an Iberia flight from Madrid to Frankfurt, forcing the pilot to turn the plane around so he could be removed, the airline said.
"A German passenger took all his clothes off on the plane," an Iberia spokeswoman said.
"Staff on board tried to dissuade him but he became aggressive and finally locked himself in the toilet.
The pilot then decided to turn around and land in Madrid."
Once on the ground, police came on board and "took him away," the spokeswoman said.
She said the airline has "no idea" why the man, who was not identified, decided to do what he did.
The passenger cooperated immediately when Guardia Civil police arrived and he was not arrested, said police spokesman Angel Blanco.
The Iberia pilot filed a complaint saying the passenger refused to obey crew instructions to sit down during the flight and then started shouting.
But the complaint did not mention him stripping naked.
Police said the man was not drunk and they had no explanation for his behaviour.