Objective: To evaluate the use of CDK4/6 inhibitors as a risk factor for medication-related osteonecrosis of the jaw (MRONJ) in a cohort of patients with metastatic breast cancer treated with denosumab. Methods: multicentre, retrospective, observational study. All patients with breast cancer treated with denosumab (January 2011-December 2022) were included. The relationship between CDK4/6 inhibitors and MRONJ was analysed. Results: 243 patients were included, ninety-five (44.2%) used a CDK4/6 inhibitor. There were 21 patients with MRONJ. In patients treated with denosumab without CDK4/6 inhibitors the incidence of MRONJ and mean time to the occurrence of MRONJ was 6.6% (8/120) and 16.8 months (SD 7.8) respectively. In patients treated with denosumab and CDK4/6 inhibitor 13.7% (13/95) and 15.4 month (SD 8.7) respectively. The difference in the incidence was not significant (p=0.085). Conclusion: it suggest that the incidence of MRONJ in patients with metastatic breast cancer treated with denosumab was higher and the onset of MRONJ occurred earlier in the presence of CDK4/6 inhibitors. Although the difference was not significant, given that the use of this combination is very common in routine clinical practice, it would be advisable to carry out larger prospective studies to clarify the risk of this association.