Radiological evidence of lytic bone lesions has a wide differential diagnosis including metastatic bone disease, multiple myeloma, primary bone cancers and infection. Here, we present the case of a woman in her 80s found to have lytic bone lesions associated with an IgA paraproteinaemia who was thus presumed to have a diagnosis of multiple myeloma. However, bone marrow biopsy results were indicative of monoclonal gammopathy of undetermined significance. She was subsequently referred to the 'carcinoma of unknown primary (CUP) team', and ultimately diagnosed with metastatic breast cancer following a repeat trephine biopsy. A range of conditions may present with lytic bone lesions and these differentials must be suitably investigated even in the presence of paraproteinaemia. Early CUP team involvement is pivotal to ensure appropriate clinical management and patient support.