Immobilization may lead to severe bone loss. Physical activity decreases with age and lower mobility might influence bone loss. We have evaluated the degree of mobility and parameters of bone turnover in 70 residents of a nursing home (mean age +/- SD 81 +/- 9 years) and 68 residents of an old people's home (mean age +/- SD 84 +/- 6 years). The mobility was assessed with a standing and walking score from 1 (severely disabled) to 5 (standing/walking without help). When the subjects were arranged according to increasing walking score from 1 to 5, fasting urinary hydroxyproline/creatinine ratio gradually decreased from 31 +/- 19 to 14 +/- 5 mumol/mmol (P less than 0.001). There was also a significant linear decrease of fasting urinary calcium/creatinine ratio, and serum calcium concentration and a significant increase of serum albumin and 1,25-dihydroxyvitamin D concentrations with increasing walking score. These data indicate that lower mobility in the elderly leads to higher bone resorption, which may suppress the formation of 1,25-dihydroxyvitamin D.