Iliac crest bone biopsies from nine children (6-15 years old) with osteogenesis imperfecta tarda (O.I.) have been studied by bone histomorphometry after double fluorescent labeling with tetracycline and compared to five unlabeled biopsies from normal children in the same age group. The results indicate that children with O.I. have a low trabecular bone volume associated with an increased bone turnover rate. Bone formation is increased at the tissue level despite a decrease in the activity of individual osteoblasts. The original defects in O.I. seem to be due to the altered rate of matrix synthesis by osteoblasts. It is, however, compensated by an increase in the number of these cells. These results suggest that these children were not losing bone at the time of the biopsy, which fits with the clinical stability of O.I. with age. Our study therefore suggests that the osteopenia observed in O.I. is most likely due to an inability to accumulate bone during growth, as normal children do, rather than to a progressive net loss of bone.