In accordance with recent terminology, it is proposed that malignant mesenchymoma should be renamed ‘composite sarcoma’ and defined as ‘a sarcoma composed of two or more cellular types each of which is sufficiently differentiated to permit clear recognition of its histogenetic type microscopically, immunohistochemically or ultrastructurally; excluding fibrosarcomatous and high-grade pleomorphic undifferentiated sarcomatous component, dedifferentiated sarcoma and the combination of osteosarcoma and chondrosarcoma which is regarded as a single histogenetic type’. Four cases of primary osseous composite sarcoma (POCS) were identified among 928 primary bone sarcomas. Their age ranged from 10 to 87 years, peak incidence in the second decade with equal sex distribution. Most presented with pain, commonest in the knee, affecting the metaphysis, appearing radiologically as expansile infiltrative osteolytic lesions with cortical erosion, periosteal reaction, variable extent of osteoblastic areas and soft tissue extension. All contained variable amounts of conventional high-grade osteosarcoma with or without chondrosarcoma component; the other constituents were liposarcoma, rhabdomyosarcoma and leiomyosarcoma. In all cases, Ki67 proliferative index was over 35%, there was no CDK4 and MDM2 amplification. The absence of low-grade component supported the de novo origin of POCS rather than derivation from divergent dedifferentiation. The two older patients with hitherto undescribed osteoleiomyosarcoma died 2 and 10 months after operation, whereas the two younger with osteorhabdomyosarcoma and osteoliposarcoma enjoyed disease-free survival at 16 and 6 years after chemotherapy despite the latter showing lung metastasis at presentation. Identification of the different lines of differentiation together with their approximate amounts and histological grades is therefore mandatory for POCS as multi-agent chemotherapy catered for each sarcoma component might offer hope for long-term disease-free survival.