Vitamin D plays a critical role in bone metabolism and many cellular and immunological processes. Serum 25-hydroxyvitamin D3 [25(OH)D3] is produced in the skin in response to exposure to ultraviolet radiation, and is a good indicator of vitamin D status. The aim of this study was to determine summer/ winter differences in serum 25(OH)D3 and Parathyroid Hormone (PTH) in Saudi pre and postmenopausal women and in adult males. We focused on the interaction of Vitamin D with other biochemical markers of bone metabolism like PTH, ALP and Calcium and how it varies on seasonal changes. The subjects were 115 Saudi adults divided into three groups; 31 males, 42 premenopausal and 42 postmenopausal women. They were medically examined twice, in summer (April-July 2014) and winter (December 2014-February 2015) months. 25(OH)D3, PTH, TSH, ALP, calcium and inorganic phosphorus were estimated and found that status of vitamin D varies significantly with seasonal changes (highest being in summer), age and also with sex. Seasonal differences in the 25(OH)D3 concentrations in these subjects appeared to reflect ability to produce 25(OH)D3 photochemically in the skin. In addition to vitamin D, levels of biochemical markers exhibited summer/winter differences and also varied among the pre and postmenopausal women and were statistically different from males.