Plasma cell tumours of the mastoid are rare malignancies of haematological origin. Two patients are described with a plasma cell tumour in the mastoid bone. In one patient it concerned a solitary plasmacytoma of the bone and in the other a focus in the mastoid of a recently diagnosed multiple myeloma. Symptoms in these patients were non-specific. Computed tomography (CT) and magnetic resonance image (MRI) scanning showed a nonspecific space-occupying lesion in the mastoid. The diagnosis was made on immunohistological examination, that showed diffuse sheets of monoclonal plasma cells in a clear matrix. When a plasmacytoma lesion is found multiple myeloma should always be excluded. Treatment in case of solitary plasmacytoma of the mastoid bone consists of radiotherapy. In case of localization in the mastoid of multiple myeloma the treatment consists of palliative chemotherapy.