Using the Cox proportional hazards model, and considering tumour necrosis and vascular invasion by tumour as additional factors in assessment, a series of 88 patients with primary Grade III and IV soft tissue sarcomas of the locomotor system was analysed for factors associated with death due to the tumour. Grade IV malignancy, a tumour size larger than 10 cm., tumour necrosis, and vascular invasion by tumour cells were significant risk factors. Patients with 0 or 1 risk factor, one half of the cases, had a 3 year survival rate of more than 90%, whereas the figure was 65% for those with 2 risk factors and 20% for those with 3 or 4 risk factors.