The histologic heterogeneity of osteoporosis relative to normal controls has attracted great interest. There has been controversy as to whether patients with high turnover osteoporosis may convert to a normal or low turnover form, and vice versa. We have studied 44 patients over 12 years by dynamic histomorphometry and 85Sr kinetics+calcium balance performed within 60 days in 20 patients (Group 1) and 75-808 days apart in the remainder (Group 2). In the first group, the histologic tissue level bone formation rate (BFR/BV or BFR/BS) was predictive of the 85Sr measurements of bone formation (r = 0.66 P < 0.01). There was no statistically significant correlation in Group 2 and the regression coefficients were significantly different (P = 0.01). Periodic regression was used to determine if seasonal changes were responsible for this loss of correlation; none was found that was of statistical significance. No systematic changes with time in bone formation were found in Group 2 during the period of observation; nor were consistent secular changes detected when the data for both groups were examined according to procedure date. In conclusion, bone formation may change with time in postmenopausal osteoporosis. Evidence that these changes are systematic was not found and this has implications for the design of treatment studies.