We have previously shown that idiopathic juvenile osteoporosis (IJO) is characterized by a decreased cancellous bone volume and a very low bone formation rate on cancellous surfaces. Whether IJO similarly affects cortical bone is unknown. We therefore compared tetracycline double-labeled transfixing iliac-crest bone biopsies from eight children with typical clinical features of IJO (six girls; age 10-12 years) and from nine children (four girls; age 9-12 years) without metabolic bone disease. No differences in intracortical remodeling activity were detected. Both structural parameters reflecting intracortical remodeling (cortical porosity, active canal diameter, and quiescent canal diameter) and bone surface-based metabolic parameters (osteoid, osteoblast, mineralizing, osteoclast and eroded surfaces, and bone formation rate) were similar in IJO patients and controls (p > 0.2 each, t-test). Although the internal cortex of the biopsy was thinner in IJO patients than in controls (660 +/- 170 microm vs. 980 +/- 320 microm; p = 0.02), there was no difference in the width of the external cortex (p = 0.36). In growing children, both cortices exhibit an external modeling drift. Therefore, the difference in internal cortical width point to a decreased modeling activity on the endocortical surface of the internal cortex. In fact, bone formation rate on this surface was 48% lower in IJO patients than in controls (82 +/- 45 microm(3)/microm(2) per year vs. 159 +/- 162 microm(3)/microm(2) per year). However, this difference did not achieve statistical significance (p = 0.21) due to the high variability of bone formation rate on modeling surfaces. The disturbance of bone remodeling in IJO is limited to cancellous bone, but there may be a modeling defect affecting the internal cortex. Thus, the process causing IJO appears to mainly affect bone surfaces that are in contact with the bone marrow cavity.