Background Diabetes mellitus is a major disease worldwide. In Saudi Arabia, it is considered to be the most common disease in the country. Diabetes mellitus has been also found to be associated with 25(OH)D (vitamin D) deficiency. In Saudi Arabia, sunlight is considered a major source for vitamin D. Saudi Arabia is popular for sunny weather most of the year, in which people can get vitamin D from the sun. However, vitamin D deficiency is common in Saudi Arabia, and its deficiency can increase blood glucose levels. We conducted a study to determine the reason for vitamin D deficiency in Saudi Arabia and to assess the relationship of diabetes mellitus with vitamin D. Aim of the work This study is aimed to assess the incidence of vitamin D deficiency in non-diabetic and type II diabetic patients in the King Faisal University (KFU) Health Center in the Al-Ahsa region. Methods Open Access Original Article