Bone width (BW), bone mineral content (BMC), and their ration (BMC/BW ratio) were measured in renal patients using direct photon absorptiometry. Serial measurements were made on the radius and ulna in 74 children with renal diseases. Values were compared to age-, sex-, height-, and weight-matched controls. The SD from the mean in normal subjects is +/- 10%. Significant demineralization (greater than -2 SD) was found in 42% of all patients and in 75% with tubulointerstitial disease. Twelve patients with nephrotic syndrome and two with systemic lupus erythematosus, all of whom were receiving prednisone therapy and had a serum creatinine level less than 1.0 mg/dl, and three treated with anticonvulsants had significant demineralization. Severe demineralization (greater than -3 SD) was found in four rachitic patients with tubulointerstitial disease. Normal mineralization was present in 32 patients with various primary glomerular diseases, seven of whom had a serum creatinine level greater than 1.5 mg/dl. BMC declined with daily prednisone therapy but increased with alternate-day dosage in seven patients. This study suggests that demineralization is more common in patients with tubulointerstitial disease and in patients with primary glomerular disease who are receiving prednisone (16 patients) or anticonvulsants. Photon absorptiometry appeared more useful than conventional radiographic evaluation in assessing skeletal involvement in childhood renal disease.