Context: Serum 25-hydroxyvitamin D [25(OH)D] may influence serum PTH and other parameters of bone health up to a threshold concentration, which may be between 25 and 80 nmol/liter. Objective: The aim of the study was to assess the threshold serum 25(OH)D with regard to PTH, bone turnover markers, and bone mineral density (BMD). Design and Setting: This was part of the Longitudinal Aging Study Amsterdam, an ongoing cohort study. Participants: A total of 1319 subjects (643 men and 676 women) between the ages of 65 and 88 yr participated in the study. Main Outcome Measures: Serum 25(OH)D, PTH, osteocalcin, urinary deoxypyridinoline/creatinine, quantitative ultrasound of the heel, BMD of lumbar spine and hip, total body bone mineral content, and physical performance. The relationship between the variables was explored by analysis of covariance and the locally weighted regression (LOESS) plots. Results: Serum 25(OH)D was below 25 nmol/liter in 11.5%, below 50 nmol/liter in 48.4%, below 75 nmol/liter in 82.4%, and above 75 nmol/liter in 17.6% of the respondents. Mean serum PTH decreased gradually from 5.1 pmol/liter when serum 25(OH)D was below 25 nmol/liter to 3.1 pmol/liter when serum 25(OH)D was above 75 nmol/liter (P < 0.001) without reaching a plateau. All BMD values were higher in the higher serum 25(OH)D groups, although only significantly for total hip (P = 0.01), trochanter (P = 0.001), and total body bone mineral content (P = 0.005). A threshold of about 40 nmol/liter existed for osteocalcin and deoxypyridinoline/creatinine, 50 nmol/liter for BMD, and 60 nmol/liter for physical performance. Conclusions: Low serum 25(OH)D concentrations are common in the elderly. Bone health and physical performance in older persons are likely to improve when serum 25(OH)D is raised above 50-60 nmol/liter.