Thirty-one patients scheduled for long-term (24 weeks) treatment with prednisone in comparatively high doses were randomly allocated to two further treatment groups. Group A received prednisone plus 'triple-treatment' (vitamin D2 45000 iu twice weekly, sodium fluoride 50 mg and calcium phosphate 4.5 g daily), group B received only prednisone. The study was undertaken in order to evaluate the effect of prednisone- and triple-treatment upon bone mineral content (BMC) and vitamin D metabolism. The groups were comparable with regard to age, sex and prednisone dose. BMC fell rapidly and similarly in both groups, demonstrating that the triple-treatment has no preventive effect on corticosteroid induced osteopenia. Serum concentrations of 25OHD2, 25OHD3 and 1,25(OH)2D were unchanged in group B (without triple-treatment), whereas in group A 25OHD2 increased enormously, 25OHD3 was suppressed possibly by substrate competition for hydroxylation in the liver and 1,25(OH)2D was halved. The suppression of 1,25(OH)2D may be an effect of raised 25OHD2 alone, or in combination with corticosteroid excess.