The symptoms of rickets due to vitamin D deficiency had been received for centuries, but the underlying cause of this condition remained unclear for a long time. The breakthrough came at the turn of the 19th and 20th centuries, when it was recognised that a deficiency of a specific substance, together with reduced sunlight exposure, both played a role in the development of rickets. First in 1922, Elmer McCollum identified the antirachitic substance, the fourth one discovered among the vitamins namely the vitamin D. Studies proved that this vitamin was essential for normal bone development and for maintaining normal serum calcium levels, but in the second half of the 20th century, a growing number of extra-skeletal effects of the vitamin was demonstrated. In the 1980s, it was a significant observation that areas with reduced sunlight exposure had higher colorectal carcinoma mortality. It started many studies, which demonstrated that the lower the serum vitamin D level was, the more frequent cancers developed and the poorer their prognosis was.  Kulcsszavak: D-vitamin, rachitis, UV-sugárzás, carcinogenesis, tumor