OBJECTIVE The aim of the present study was to measure concentrations of vitamin D metabolites and intact parathyroid hormone (iPTH) serum concentrations and an urinary bone resorption marker in two groups of elderly subjects, who differed markedly in their sedentary status and seasonality. METHODS 193 institutionalized elderly people of a long-stay geriatric ward (106 women: mean age 82; 87 men: mean age 78) were studied during wintertime. 312 ambulatory elderly people (109 women: mean age 74; 203 men: mean age 76) were studied during summertime. Concentrations of calcidiol (25(OH)D), calcitriol (1,25(OH)2D) and serum iPTH, as well as urinary N-telopeptides (NTX) were measured. RESULTS Vitamin D deficiency (defined as serum 25(OH)D < 12 ng/ml) was present in 86% of institutionalized at the expected nadir (wintertime), compared to 15% of the ambulatory elderly subjects at the expected maximum (summertime). Serum calcitriol concentrations were significantly lower in institutionalized subjects (p = .0001). However intact PTH concentrations did not differ significantly between institutionalized and ambulatory elderly. Institutionalized and female subjects showed higher urinary NTX excretion (female institutionalized: 131.9; female ambulatory: 66.8/male institutionalized: 76.3 male ambulatory: 45.8 nmol/mmol). CONCLUSIONS This cross-sectional study documented very low serum calcidiol and calcitriol concentrations and high urinary N-telopeptide excretion in institutionalized elderly people. There was no difference in serum iPTH concentrations between institutionalized and ambulatory elderly. This finding could not be explained by the differences in calcidiol and calcitriol concentration, nor urinary NTX excretion. These results suggest that other factors than vitamin D deficiency, such as lower mobility status and sedentary life style, might have an important role in the regulation of iPTH and mechanisms of bone loss in the elderly.