It appears that the so-called mesothelium lining the serous cavities is in fact mesoderm, with an epithelial-like arrangement which persists in the post-embryological stage. Cellular variability of primary tumours of serous surfaces can be explained on this developmental evidence. The term mesodermoma seems to be justified to define neoplasms arising from undifferentiated and multipotential mesoderm which, in neoplastic disease, exhibits a wide range of differential activity and gives rise to tumours with prevailing myoblastic, angioblastic, lymphoblastic, chondroblastic, osteoblastic, fibroblastic or epithelial-like features.