We have compared the mean wall thickness (MWT) and active formation periods (sigma f(A] of trabecular bone packets in iliac crest biopsies from 20 patients (7 male, 13 female) with corticosteroid-induced osteoporosis (CS-OP) and 20 age- and sex-matched controls. The trabecular bone volume (TBV) of the CS-OP patients (9.6% +/- 2.2% [SD]) was significantly reduced compared to controls (19.3% +/- 5.1%). The MWT of CS-OP patients (32.7 +/- 4.3 micron) was also significantly lower than the control value (48.0 +/- 6.2 micron). There was a positive correlation between MWT and TBV in both groups. The mineralization rate (M) of the CS-OP patients (0.54 +/- 0.25 micron/day) was within the normal range, and since there was no increase in osteoid seam thickness, so therefore was the osteoblastic appositional rate (OAR). The active formation period of trabecular bone packets (sigma f(A) = MWT/M) was significantly lower in the CS-OP patients (55.9 +/- 14.4 days) than in the control group (68.1 +/- 9.4 days). MWT and sigma f(A) both decreased with age in the control group, whereas in the CS-OP group they were independent of age. We conclude that corticosteroid therapy results in a reduction of the MWT of trabecular bone packets and, consequently, of TBV. In these patients, where the OAR was normal, the reduction in MWT was apparently caused by a shortening of the life-span of the active osteoblastic population at the basic multicellular unit (BMU) level.