Nitrogen-bisphosphonates (n-BP), often referred to as aminobisphosphonates, are the most commonly prescribed drugs for the treatment of disorders of bone fragility. However, long-term continuous treatment predisposes certain individuals to serious rare side effects, such as bisphosphonate-associated osteonecrosis of the jaw (BAONJ). n-BP use is known to unintentionally activate a subset of innate T cells called Vg9Vd2 T cells, but the consequence of this chronic immune stimulation has remained unexplored. The primary objectives of this study were to 1) determine the fate of Vg9Vd2 T cells in osteoporotic patients on n-BP therapy as a function of time and type of therapy; 2) evaluate the proportion of Vg9Vd2 T cells in patients who had recently experienced n-BP-associated ONJ. We found there is a notable loss of Vg9Vd2 T cells over time in osteoporotic patients on n-BP therapy, particularly those on intravenous (iv) therapy (Spearman r ¼ À0.55, p < 0.0001 iv; r ¼ À0.3, p < 0.03 oral) (n ¼ 68); no difference was observed in total T cells, monocytes, or granulocytes. Importantly, the observed negative effect on Vg9Vd2 T cells coincides with the reported route of administration and timing of the rare occurrence of BAONJ. Patients (n ¼ 6) who had experienced BAONJ were all found to be significantly deficient in Vg9Vd2 T cells (median ¼ 0.07%) in comparison to age-and sex-matched treatment-naïve controls (N ¼ 11; median ¼ 2.40%), U ¼ 0, p ¼ 0.001; this was the only consistent difference in the leukocytes assessed. All BAONJ cases had an underlying condition that further contributed to impaired immunity. We propose Vg9Vd2 T cells show a strong potential to serve as harbingers of possible adverse immune effects of n-BP therapy, particularly in those patients already having a compromised immune system as they may be most vulnerable to the development of conditions such as BAONJ. ß