Hypoparathyroidism, a disorder in which PTH is absent, is associated with BMD that is above average. We studied associated structural and dynamic properties of the skeleton in hypoparathyroidism. Thirty-three subjects with hypoparathyroidism and 33 age- and sex-matched control subjects with no known metabolic diseases underwent percutaneous iliac crest bone biopsies after double-labeling with tetracycline. The main outcome was histomorphometric assessment of structural and dynamic skeletal parameters. Subjects with hypoparathyroidism had greater cancellous bone volume (mean +/- SD; BV/TV: 23.5 +/- 8 versus 19.7 +/- 5%, p = 0.02), trabecular width (Tb.Wi: 136.1 +/- 37 versus 119.3 +/- 21 microm, p = 0.03), and cortical width (Ct.Wi: 923.4 +/- 420 versus 753.5 +/- 246 microm, p = 0.05) than control subjects. Dynamic skeletal indices, including mineralizing surface (MS: 0.85 +/- 1.58 versus 4.27 +/- 3.32%, p < 0.0001) and bone formation rate (BFR/BS: 0.006 +/- 0.014 versus 0.032 +/- 0.028 microm(3)/microm(2)/d, p < 0.0001), were profoundly suppressed in the hypoparathyroid subjects. We conclude that hypoparathyroidism is characterized by markedly unusual structural and dynamic properties of bone.