The mean wall thickness (MWT) and duration of formation periods (sigmaf) of trabecular bone packets have been measured in iliac crest biopsies following double tetracycline labeling from 9 women having primary osteoporosis, with vertebral crush fractures and reduced trabecular bone volume (TBV), and 9 age- and sex-matched controls. The MWT of the osteoporotic biopsies was significantly less than that of the controls and was negatively correlated with age in the latter. There was also a positive correlation between MWT and TBV in the controls but not in the osteoporotics. Sigmaf, in days, showed a tendency to decline with age in the control biopsies and was further decreased in the osteoporotic patients. These results suggest that a major contribution to the negative skeletal balance existing in both primary osteoporosis and physiological osteopenia is a decrease in bone formation, caused by a reduction in the life span of the osteoblastic population at the basic multicellular unit (BMU) level.