We describe an unusual bone-marrow metastasis of Merkel cell carcinoma (MCC) arising in the right cheek of a 73-year-old woman with systemic lupus erythematosus (SLE) and Sjögren's syndrome, who had been treated with oral prednisolone and methotrexate for 10 years. Seven months after wide local excision followed by local irradiation, the patient presented with thrombocytopaenia. Her bone marrow had been completely replaced by metastatic MCC cells, and metastatic cytokeratin 20-positive cells were also identified in the peripheral blood. To our knowledge, in the English literature, only six cases have been described previously of MCC bone-marrow involvement. Of these six cases, four were immunosuppressed, similar to our case. The high incidence of MCC in immunosuppressed patients such as those with SLE has been discussed previously. We consider that immunosuppression might be associated with bone-marrow metastasis, which is a rare form of MCC.