Hotel Koldingfjord's original name was "Julemærkesanatoriet" (Christmas seal sanatorium). Postmaster Einar Holbøll, had the idea of selling Christmas seals. The idea was that everyone, rich as well as poor, should be involved in helping weak and ill children. When people sent Christmas cards to each other, they gave 2 ører extra for a Christmas seal, which was glued onto the envelope next to the stamp. In so doing, people contributed to financing the construction of several sanatoriums. Hotel Koldingfjord was the first Christmas seal sanatorium constructed in Denmark.
The sale of Christmas seals started in 1904 and brought in DKK 67,000. It was decided that the money should be used for the construction of a sanatorium for children with pulmonary tuberculosis. After much toing and froing, the property, Louisehøj, with the adjacent hills in Kolding was bought. The Julemærkesanatoriet was constructed in the years 1907-1911. The construction was budgeted at DKK 600,000 but the final price was DKK 1 million. This was due to a great many significant problems with the subsoil, which was very soft. It was necessary to create stable subsoil by driving piles into the soil. This means driving poles into the soil in order to prevent the building from sliding out into the fjord. It is said that there is just as much construction below the ground as above the ground!
From 1911 to 1960, the buildings functioned as a sanatorium for children with tuberculosis and thus became an important piece in the battle against the feared and widespread disease.
The buildings had various functions. Where the terrace is located today, there was an outdoor bed unit. The conference rooms were the kitchen, dining room for the children and employees, "bathing passage" to the sailboats as well as a woodwork room. The reception was fitted out as a large living room, in the "100 hallway" there were doctors' rooms and x-ray room as well as large sick wards. On the 1st floor there were small rooms in the middle and sick wards on each side. The operating room and dental clinic were on the uppermost floor in the middle and likewise, sick wards on each side.
The building behind the main building, where the Magnolie room and Bøge room are today, was originally a gymnastics hall and boiler section, whilst the side buildings, which today are called Dagmar's Palace and Christian's Palace, were used as a school and office.
After 1960, the buildings were taken over by Åndsvageforsorgen (The Care of Mentally Disabled) in Vejle County. From 1983 to 1987, the majority of the buildings stood empty until Sygeplejerskernes Ejendoms Aktieselskab acquired the buildings with a view to building a modern hotel and conference centre. As an institution, the Julemærkesanatoriet's time was thus over in 1983.
From 1988 to 1990, the listed buildings were renovated and rebuilt. Internally, the property has gone through a total renovation and modernisation and it has been equipped with everything that is part of a modern hotel and conference centre. On 1 May 1990, the work was completed and the property opened under the name, Hotel Koldingfjord.