Background: Few studies have examined the variation in oestradiol with respect to age and circadian rhythm and the subsequent effects on BMD. Aim: Demonstrate the presence or absence of a circadian rhythm for oestrodial in older men and the integral role of concerted circadian rhythms of several factors including parathyroid hormone (PTH) in regulating biochemical markers of bone resorption and formation. Examine whether concentrations of both circulating total and bioavailable oestrogen in men differ with age and BMD. Design: Males were recruited: young men with normal BMD, older men with normal BMD and older men with osteoporosis. Methods: Subjects were hospitalized for a 25-hour period. Blood samples were obtained every 30 minutes. Hormone analysis results were plotted and reviewed. Results: Both total and bioavailable oestradiol concentrations were significantly lower in the older men than the young men (Total oestradiol: 34.5±4.4 pmol/L vs. 49.0±6.5 pmol/L, p<0.0001; Bioavailable oestradiol 16.7±2.2 pmol/L vs. 26.3±3.6 pmol/L, p<0.0001). Bioavailable oestrogen rhythm mirrored that of total estrogen. Conclusion: Both age groups with normal BMD display circadian rhythmicity with respect to circulating and bioavailable oestradiol. Younger men have increased mean total and bioavailable oestrogen concentrations and later acrophase compared to older counterparts. In older men with low BMD, total circulating oestrogen was not significantly different compared to age-matched older men with normal BMD; bioavailable oestrogen was significantly lower. Total oestrogen demonstrated a concerted circadian rhythm in all 3 groups, but bioavailable oestrogen did not demonstrate circadian rhythmicity in older men with decreased BMD.