The 50-year-old prince, 11th in line to the throne, was rushed to hospital a week ago suffering from pneumonia and has been "placed in an artificial coma to optimise the treatment", said palace spokesman Pierre-Emmanuel de Bauw.
His condition "is stable", the spokesman added.
King Philippe, the oldest son of Albert II who abdicated in his favour last year, visited Laurent yesterday while Queen Mathilde went to the hospital today.
Fined several times for speeding and chided for fraternising with African leaders without the government's consent, Laurent has long been considered the "enfant terrible" of the Belgian royal family.