Seventy children with juvenile chronic arthritis have had measurements of cortical and trabecular bone density in one or both radii. In 7 children with unilateral disease of one wrist, there was a substantial reduction in growth on the affected side. Trabecular bone density in the distal radius was reduced in the main group of 63 patients compared with controls, and this deficit was appreciably worse if the wrist was clinically affected by disease or if the child was being treated with steroids. Cortical bone density in the midshaft was less affected. Crush fractures of the spine were associated with more prolonged periods of bed rest, steroid therapy, radial trabecular bone density more than 2 standard deviations below normal, and subnormal 25-hydroxycholecalciferol concentrations in the serum. Since steroid therapy is often mandatory the main therapeutic implications are that the more severely affected child often needs vitamin D supplementation in "physiological" dosage, and that early mobilisation and reduction of steroid dosage should be constant aims.