BACKGROUND Many authors have considered dental implants to be unrelated to increased risk of medication-related osteonecrosis of the jaw (MRONJ). Nevertheless, more recently, more cases of peri-implant MRONJ (PI-MRONJ) have been described, thus becoming a challenging health problem. Also, metastatic cancer deposits are not infrequently found at peri-implant sites and this may represent an additional complication for such treatments. We present the case of a breast cancer patient with PI-MRONJ, presenting a clinically and radiologically undetected metastasis within the necrotic bone, and highlight the necessity of an accurate histopathological analysis. CASE REPORT A 66-year-old female patient, who had received intravenous bisphosphonates for bone breast cancer metastases, came to our attention for a non-implant surgery-triggered PI-MRONJ. After surgical resection of the necrotic bone, conventional and immunohistochemical examinations were performed, which showed breast cancer deposits within the necrotic bone. CONCLUSIONS Cancer patients with metastatic disease, who are undergoing bisphosphonate treatment, may develop unusual complications, including MRONJ, which is a site at risk for hosting additional metastatic deposits that may be clinically and radiologically overlooked. Such risk is increased by previous or concomitant implant procedures. Consequently, clinicians should be prudent when performing implant surgery in cancer patients with advanced-stage disease and consider the possible occurrence of peri-implant metastases while planning adequate treatments in such patients.