The mean wall thickness of iliac trabecular bone packets was determined in undecalcified iliac crest bone specimens from 14 hypothyroid patients, 14 hyperthyroid patients and 28 age- and sex-matched normal controls. Calcification rate and fractional resorption, formation and labeled surfaces in trabecular bone from the patients were measured by conventional histomorphometric methods after double-labeling with tetracycline. The mean wall thickness was increased in the hypothyroid patients (58,9 μm), while no change was demonstrated in the hyperthyroid group (52,5 μm) as compared to the controls (54,0 μm). Calculated estimates of trabecular bone remodeling at the "Basic Multicellular Unit" level showed longer periods of resorption, formation and total remodeling in the hypothyroid than in the hyperthyroid patients.