Thailand, a Southeast Asian country, is located at latitudes between 5°30' N and 20°30' N. Vitamin D intake among Thais is generally low because few natural vitamin D-rich food sources are found in Thailand, and foods are not fortified with vitamin D. Up to now, there has been a lack of reliable epidemiological data concerning vitamin D status in Thais. Therefore, the purpose of this study is to investigate vitamin D status in Thais according to geographical region by assessing levels of serum 25-hydroxyvitamin D (which is the major metabolite and represents the stored form of vitamin D) by a reference method, liquid chromatography/tandem mass spectrometry (LC-MS/MS).