Abstract The bone tissue formation rate, the mean osteon formation period, and the number of new osteons introduced into cortical bone per year were measured in bone biopsies from 21 adult patients with osteoporosis of diverse etiology and were compared to normal patients. The study involved in vivo tetracycline bone labeling, subsequent biopsy of the anterior 5 cm. of the eleventh rib, and quantitative histological analyses of mineralized bone sections made from the biopsies. With respect to the bone formation rate, we found: (1) both high and low but usually low rates in postmenopausal and senile osteoporosis, (2) a significant increase in 3 cases of adult osteogenesis imperfecta, and (3) a significant increase in one case of active acromegaly. In the osteon formation time, we found a significant increase in postmenopausal and senile osteoporosis, high normal values in osteogenesis imperfecta, and a high normal value in active acromegaly. In the number of new osteons formed each year, we found: (1) variable but preponderantly low values in postmenopausal and senile osteoporosis, (2) significant increases in osteogenesis imperfecta, and (3) a high normal value in acromegaly.