Bone mineral density (BMD) was studied in 26 children with idiopathic nephrotic syndrome and in age- and sex-matched healthy controls. BMD was selectively measured in trabecular (TBD), cortical (CBD) and total bone (BD) using peripheral quantitative computed tomography. Patients showed a decrease in BD, CBD and TBD. BD and CBD were inversely correlated with the cumulative dose of steroid treatment. Of the 26 patients with high cumulative doses of steroid, 16 were also treated with cyclophosphamide. In this group BD and CBD were decreased significantly compared with the children with a low cumulative steroid dose only. Compared with controls for each subgroup, significant decreases in BD, CBD and TBD were found in the group with high cumulative doses of steroids only. The higher cumulative steroid dose and the initial steroid toxicity which made cytotoxic therapy necessary, rather than cyclophosphamide itself, may be responsible for these findings.