Danish Prince Joachim, the youngest son of Queen Margrethe II, has undergone an operation for a blood clot in the brain in France, the palace said on Saturday.
The operation, described as successful, took place at the University Hospital of Toulouse.
The prince was admitted late Friday, and the palace statement said he was in a stable condition.
The 51-year-old prince moved to France last year with his French-born wife Princess Marie and their two children after he was invited to attend France's highest ranking military leadership training course at the École Militaire defence college in Paris.
He graduated last month.
Earlier in June, he was appointed military attache at the Danish embassy in Paris. He was set to begin his three-year appointment at the embassy in September.
Sixth in line to the throne
Joachim is sixth in line to the throne, following his older brother, Crown Prince Frederik, and Frederik's four children.
The palace said the queen hoped Joachim's privacy would be respected while he was in hospital. The palace said Marie, who married Joachim in 2008, was with her husband.
A few days ago, Joachim and other members of his family were at the queen's estate, Chateau de Cayx, in Cahors, southern France.
They celebrated the 18th birthday of Prince Felix, one of Joachim's two sons from his previous marriage to Countess Alexandra, a photo posted on Instagram showed.