Age-related alterations within the vitamin D endocrine system have been implicated in the pathogenesis of bone loss and osteoporosis. Compromises in vitamin D status and/or metabolism of vitamin D to the hormonal form, 1,25dihydroxyvitamin D (1,25(OH)2D) are thought to be responsible for a decline in calcium absorption which in turn leads to compensatory hypersecretion of parathyroid hormone (PTH) and enhanced osteolysis in order to maintain normocalcemia. It has been hypothesized that compromised vitamin D status and/or circulating levels of the active, hormonal form of the vitamin are a concomitant of aging and are responsible for part of the bone loss seen in the elderly. To test this hypothesis normal men and women from the BLSA and elderly patients who have not been exposed to sunlight for 6 months have been studied. There were no differences across the age span in 25-OHD or 1,25(OH)2D levels in men or women. Both men and women had an age associated increase in PTH levels that was not independently correlated with creatinine clearance. While women had no change with age in ionized calcium or phosphorus levels men had a slight (4%) decline in ionized calcium and a marked (25%) decline in phosphorus. In both sexes there was a seasonal variation in vitamin D and in females a variation in 1,25(OH)2D. The sunlight deprived patients had significantly lower vitamin D levels than controls, and 43 % had levels <15 ng/ml with 30% <10. None of the control population were <10.