Serum-1,25-dihydroxyvitamin-D3 (1,25-[OH]2D3) was subnormal in children receiving long-term glucocorticoid treatment for various glomerular diseases, including nephrotic syndrome. In children with chronic glomerulonephritis not treated with glucocorticoids who had similar serum-creatinine with glucocorticoids who had similar serum-creatinine concentrations, serum-1,25-dihydroxyvitamin-D3 concentrations resembled those in healthy controls, indicating that glomerular renal disease per se does not account for reduced serum-1,25(OH)2DE concentrations in steroid-treated patients. The reduction in concentration of this most active vitamin-D metabolite correlated with the dose of steroid administered and with reduction in forearm bone mineral content measured by the photon absorption technique. Reduced serum-1,25-(OH)2D3 concentration may be important in the pathogenesis of steroid-induced osteopenia.