Histomorphometric analysis was performed on transiliac undecalcified bone biopsies taken from eight adults and four children with osteogenesis imperfecta. Double tetracycline labelling done before the biopsy allowed measurement of the calcification rate in 11 patients. The adults, as well as the children, had thin iliac cortices and considerably decreased trabecular bone volume. In addition, the calcification rate was significantly reduced. Since the thickness index of osteoid seams was low or normal in all subjects, it indicated a reduced bone formation at the cellular level. In the adults, there was no significant increase in the parameters of bone resorption or in the bone formation rates at the basic multicellular unit and the tissue levels. Thus the histomorphometric profile of iliac bone in osteogenesis imperfecta is characterised by cortical and trabecular osteoporosis with decreased activity of the individual osteoblasts. These findings would suggest the use of therapeutic agents stimulating bone formation rather than agents inhibiting bone resorption in osteogenesis imperfecta.