Primary mucoepidermoid carcinoma of the thymus developing in an 80-year-old Japanese man is described. This is the third case report of this rare tumour which was diagnosed following fine needle aspiration cytology (FNAC) of a metastatic lesion in the left humerus. FNA showed the typical cytological findings of mucoepidermoid carcinoma, that is the presence of squamous, glandular and intermediate neoplastic cells. Histology at autopsy confirmed these findings.