The immediate effect of the Boston brace on renal function was studied in 18 patients with idiopathic scoliosis ranging in age from ten to 17 years. Renal function was tested by clearances of inulin and PAH without and with brace applied. Glomerular filtration rate and effective renal plasma flow decreased significantly when the brace was applied, but to values within normal limits. When the brace was applied, the urinary sodium excretion, however, was significantly reduced to values significantly lower than that of controls. This might cause sodium retention if the pressure effect of the brace persists.