Abstract.Parameters of renal function and calcium homeostasis were studied in 8 children, immobilised for 5‐9 weeks with fractured femurs, weekly during immobilisation and fourth weekly following mobilisation until all parameters returned to normal. During immobilisation 1 patient became hypercalcaemic, but all showed an increase in serum calcium and all developed hypercalciuria. During immobilisation all showed diminished urine osomolality after a 12‐hour fast (mean 591·133 mOsm/kg) which improved 4‐39 weeks after mobilisation (mean 973·87 mOsm/kg). Serum creatinine, urinary beta‐2‐microglobulin and renal ultrasound appearances were all normal. An inverse relationship, R= ‐0.70, was demonstrated between serum calcium and fasting urine osmolality during immobilisation. Three patients showed diminished urinary concentrating ability beyond 4 weeks after mobilisation. For 1 patient this defect persisted for 8 months and glomerular filtration rate was diminished 9 months after mobilisation, raising the possibility of long term renal damage in immobil