A 78-year-old man was referred to the Special Needs Dentistry Clinic for review prior to beginning bisphosphonate treatment as part of the management for his metastatic prostate cancer given the increased risk of medication-related osteonecrosis of the jaws in this patient cohort. Imaging revealed a well-defined radiolucency in the mandible that was closely associated with a non-vital anterior tooth. The lesion was asymptomatic. Incisional biopsy revealed a low grade B cell lymphoproliferation compatible with a monoclonal B-cell lymphocytosis (MBL). The patient was subsequently referred to haematology as MBL may develop into chronic lymphocytic leukaemia. © BMJ Publishing Group Limited 2020. No commercial re-use. See rights and permissions. Published by BMJ.